Index: Core/Common/src/Core.Common.Base/ObservableUniqueItemCollectionWithSourcePath.cs
===================================================================
diff -u -r48e2db6c910f4ec279d2d63fbb1cc05a1196b7f1 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Core/Common/src/Core.Common.Base/ObservableUniqueItemCollectionWithSourcePath.cs	(.../ObservableUniqueItemCollectionWithSourcePath.cs)	(revision 48e2db6c910f4ec279d2d63fbb1cc05a1196b7f1)
+++ Core/Common/src/Core.Common.Base/ObservableUniqueItemCollectionWithSourcePath.cs	(.../ObservableUniqueItemCollectionWithSourcePath.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -170,22 +170,23 @@
             return GetEnumerator();
         }
 
+        /// <summary>
+        /// Validates the items of an <see cref="IEnumerable{TObject}"/> based on their feature.
+        /// </summary>
+        /// <param name="items"></param>
+        /// <exception cref="ArgumentException">Thrown when 
+        /// <list type="bullet">
+        /// <item>one of the items is <c>null</c></item>
+        /// <item>when a duplicate item was found.</item>
+        /// </list>
+        /// </exception>
         private void InternalValidateItems(IEnumerable<TElement> items)
         {
             if (items.Contains(null))
             {
-                throw new ArgumentException("Collection cannot contain null.", nameof(items));
+                throw new ArgumentException(@"Collection cannot contain null.", nameof(items));
             }
-            ValidateItems(items);
-        }
 
-        /// <summary>
-        /// Validates the items of an <see cref="IEnumerable{TObject}"/> based on their feature.
-        /// </summary>
-        /// <exception cref="ArgumentNullException">Thrown when any parameters are <c>null</c>.</exception>
-        /// <exception cref="ArgumentException">Thrown when a duplicate item was found.</exception>
-        private void ValidateItems(IEnumerable<TElement> items)
-        {
             IEnumerable<IGrouping<object, TElement>> duplicateItems =
                 items.Concat(collection).GroupBy(getUniqueFeature)
                      .Where(group => group.Count() > 1);
Index: Ringtoets/ClosingStructures/test/Ringtoets.ClosingStructures.Plugin.Test/FileImporters/ClosingStructureUpdateDataStrategyTest.cs
===================================================================
diff -u -r12cec002453a1828efc68633fbd25219632c6c47 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/ClosingStructures/test/Ringtoets.ClosingStructures.Plugin.Test/FileImporters/ClosingStructureUpdateDataStrategyTest.cs	(.../ClosingStructureUpdateDataStrategyTest.cs)	(revision 12cec002453a1828efc68633fbd25219632c6c47)
+++ Ringtoets/ClosingStructures/test/Ringtoets.ClosingStructures.Plugin.Test/FileImporters/ClosingStructureUpdateDataStrategyTest.cs	(.../ClosingStructureUpdateDataStrategyTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -27,17 +27,16 @@
 using Core.Common.Base.Geometry;
 using Core.Common.TestUtil;
 using NUnit.Framework;
+using Ringtoets.ClosingStructures.Data;
+using Ringtoets.ClosingStructures.Data.TestUtil;
+using Ringtoets.ClosingStructures.Plugin.FileImporters;
 using Ringtoets.Common.Data;
 using Ringtoets.Common.Data.Exceptions;
 using Ringtoets.Common.Data.FailureMechanism;
 using Ringtoets.Common.Data.Structures;
 using Ringtoets.Common.Data.TestUtil;
 using Ringtoets.Common.Data.UpdateDataStrategies;
-using Ringtoets.Common.IO.Exceptions;
 using Ringtoets.Common.IO.Structures;
-using Ringtoets.ClosingStructures.Data;
-using Ringtoets.ClosingStructures.Data.TestUtil;
-using Ringtoets.ClosingStructures.Plugin.FileImporters;
 using Ringtoets.Common.Utils;
 
 namespace Ringtoets.ClosingStructures.Plugin.Test.FileImporters
@@ -71,7 +70,7 @@
                 }).SetName("Different AllowedLevelIncreaseStorage");
                 yield return new TestCaseData(new TestClosingStructure
                 {
-                    AreaFlowApertures = 
+                    AreaFlowApertures =
                     {
                         Mean = (RoundedDouble) random.Next(),
                         Shift = random.NextRoundedDouble(),
@@ -97,15 +96,15 @@
                 }).SetName("Different FlowWidthAtBottomProtection");
                 yield return new TestCaseData(new TestClosingStructure
                 {
-                    InsideWaterLevel = 
+                    InsideWaterLevel =
                     {
                         Mean = (RoundedDouble) random.Next(),
                         StandardDeviation = random.NextRoundedDouble()
                     }
                 }).SetName("Different InsideWaterLevel");
                 yield return new TestCaseData(new TestClosingStructure
                 {
-                    LevelCrestStructureNotClosing = 
+                    LevelCrestStructureNotClosing =
                     {
                         Mean = (RoundedDouble) random.Next(),
                         StandardDeviation = random.NextRoundedDouble()
@@ -121,7 +120,7 @@
                 }).SetName("Different StorageStructureArea");
                 yield return new TestCaseData(new TestClosingStructure
                 {
-                    ThresholdHeightOpenWeir = 
+                    ThresholdHeightOpenWeir =
                     {
                         Mean = (RoundedDouble) random.Next(),
                         StandardDeviation = random.NextRoundedDouble()
@@ -291,7 +290,7 @@
             // Assert
             var exception = Assert.Throws<UpdateDataException>(call);
 
-            string expectedMessage = $"Kunstwerken moeten een unieke id hebben. Gevonden dubbele elementen: {duplicateId}.";
+            const string expectedMessage = "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(expectedMessage, exception.Message);
 
             CollectionAssert.IsEmpty(targetCollection);
Index: Ringtoets/Common/src/Ringtoets.Common.Data/UpdateDataStrategies/UpdateDataStrategyBase.cs
===================================================================
diff -u -r48e2db6c910f4ec279d2d63fbb1cc05a1196b7f1 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/Common/src/Ringtoets.Common.Data/UpdateDataStrategies/UpdateDataStrategyBase.cs	(.../UpdateDataStrategyBase.cs)	(revision 48e2db6c910f4ec279d2d63fbb1cc05a1196b7f1)
+++ Ringtoets/Common/src/Ringtoets.Common.Data/UpdateDataStrategies/UpdateDataStrategyBase.cs	(.../UpdateDataStrategyBase.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -120,10 +120,6 @@
             {
                 throw new UpdateDataException(e.Message, e);
             }
-            catch (InvalidOperationException e)
-            {
-                throw new UpdateDataException(Resources.UpdateDataStrategyBase_UpdateTargetCollectionData_Imported_data_must_contain_unique_items, e);
-            }
         }
 
         /// <summary>
@@ -138,8 +134,8 @@
         /// <returns>A <see cref="IEnumerable{T}"/> with affected objects.</returns>
         /// <exception cref="ArgumentException">Thrown when duplicate items are being added to the 
         /// <paramref name="targetDataCollection"/>.</exception>
-        /// <exception cref="InvalidOperationException">Thrown when duplicate items are found during the 
-        /// update of the items to be updated in the <paramref name="targetDataCollection"/>.</exception>
+        /// <exception cref="UpdateDataException">Thrown when duplicate items are found in the 
+        /// <paramref name="importedDataCollection"/>.</exception>
         private IEnumerable<IObservable> ModifyDataCollection(ObservableUniqueItemCollectionWithSourcePath<TTargetData> targetDataCollection,
                                                               IEnumerable<TTargetData> importedDataCollection,
                                                               string sourceFilePath)
@@ -184,12 +180,16 @@
         /// <param name="objectsToUpdate">The objects that need to be updated.</param>
         /// <param name="importedDataCollection">The data to update from.</param>
         /// <returns>A <see cref="IEnumerable{T}"/> of affected items.</returns>
-        /// <exception cref="InvalidOperationException">Thrown when the imported 
+        /// <exception cref="UpdateDataException">Thrown when the imported 
         /// <paramref name="importedDataCollection"/> contains duplicate items.</exception>
         private IEnumerable<IObservable> UpdateData(IEnumerable<TTargetData> objectsToUpdate,
                                                     IEnumerable<TTargetData> importedDataCollection)
         {
             var affectedObjects = new List<IObservable>();
+            if (importedDataCollection.Count() != importedDataCollection.Distinct(equalityComparer).Count())
+            {
+                throw new UpdateDataException(Resources.UpdateDataStrategyBase_UpdateTargetCollectionData_Imported_data_must_contain_unique_items);
+            }
 
             foreach (TTargetData objectToUpdate in objectsToUpdate)
             {
Index: Ringtoets/Common/test/Ringtoets.Common.Data.Test/UpdateDataStrategies/UpdateDataStrategyBaseTest.cs
===================================================================
diff -u -rb3b6c13cf736c134476b3db34281332d01ca86b1 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/Common/test/Ringtoets.Common.Data.Test/UpdateDataStrategies/UpdateDataStrategyBaseTest.cs	(.../UpdateDataStrategyBaseTest.cs)	(revision b3b6c13cf736c134476b3db34281332d01ca86b1)
+++ Ringtoets/Common/test/Ringtoets.Common.Data.Test/UpdateDataStrategies/UpdateDataStrategyBaseTest.cs	(.../UpdateDataStrategyBaseTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -208,9 +208,10 @@
         {
             // Setup
             var collection = new TestUniqueItemCollection();
+            var testItem = new TestItem("I am an expected item");
             collection.AddRange(new[]
             {
-                new TestItem("I am an expected item")
+                testItem
             }, sourceFilePath);
 
             const string duplicateName = "Duplicate Name";
@@ -227,11 +228,13 @@
 
             // Assert
             var exception = Assert.Throws<UpdateDataException>(call);
-            string message = $"TestItem moeten een unieke naam hebben. Gevonden dubbele elementen: {duplicateName}.";
+            const string message = "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(message, exception.Message);
-            Assert.IsInstanceOf<ArgumentException>(exception.InnerException);
 
-            CollectionAssert.IsEmpty(collection);
+            CollectionAssert.AreEqual(new[]
+            {
+                testItem
+            }, collection);
         }
 
         [Test]
@@ -469,7 +472,6 @@
             // Assert
             var exception = Assert.Throws<UpdateDataException>(call);
             Assert.AreEqual("Geïmporteerde data moet unieke elementen bevatten.", exception.Message);
-            Assert.IsInstanceOf<InvalidOperationException>(exception.InnerException);
         }
 
         [Test]
Index: Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Plugin.Test/FileImporters/GrassCoverErosionInwardsDikeProfileUpdateDataStrategyTest.cs
===================================================================
diff -u -r064653222d1d4df89ac7c644f2227f2fe0aa2070 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Plugin.Test/FileImporters/GrassCoverErosionInwardsDikeProfileUpdateDataStrategyTest.cs	(.../GrassCoverErosionInwardsDikeProfileUpdateDataStrategyTest.cs)	(revision 064653222d1d4df89ac7c644f2227f2fe0aa2070)
+++ Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Plugin.Test/FileImporters/GrassCoverErosionInwardsDikeProfileUpdateDataStrategyTest.cs	(.../GrassCoverErosionInwardsDikeProfileUpdateDataStrategyTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -182,7 +182,7 @@
             var exception = Assert.Throws<DikeProfileUpdateException>(call);
 
             string expectedMessage = "Het bijwerken van de dijkprofielen is mislukt: " +
-                                     $"Dijkprofielen moeten een unieke id hebben. Gevonden dubbele elementen: {duplicateId}.";
+                                     "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(expectedMessage, exception.Message);
             Assert.IsInstanceOf<UpdateDataException>(exception.InnerException);
 
Index: Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Plugin.Test/FileImporters/HeightStructureUpdateDataStrategyTest.cs
===================================================================
diff -u -r8c97f59adda15f1eb2a4550a06d2f7fb8bee91bf -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Plugin.Test/FileImporters/HeightStructureUpdateDataStrategyTest.cs	(.../HeightStructureUpdateDataStrategyTest.cs)	(revision 8c97f59adda15f1eb2a4550a06d2f7fb8bee91bf)
+++ Ringtoets/HeightStructures/test/Ringtoets.HeightStructures.Plugin.Test/FileImporters/HeightStructureUpdateDataStrategyTest.cs	(.../HeightStructureUpdateDataStrategyTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -216,7 +216,7 @@
         }
 
         [Test]
-        public void UpdateStructuresWithImportedData_WithoutCurrentStructuresAndReadStructuresHaveDuplicateNames_ThrowsUpdateDataException()
+        public void UpdateStructuresWithImportedData_WithoutCurrentStructuresAndReadStructuresHaveDuplicateIds_ThrowsUpdateDataException()
         {
             // Setup
             const string duplicateId = "I am a duplicate id";
@@ -237,8 +237,7 @@
             // Assert
             var exception = Assert.Throws<UpdateDataException>(call);
 
-            string expectedMessage = "Kunstwerken moeten een unieke id hebben. " +
-                                     $"Gevonden dubbele elementen: {duplicateId}.";
+            const string expectedMessage = "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(expectedMessage, exception.Message);
 
             CollectionAssert.IsEmpty(targetCollection);
Index: Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/RingtoetsPipingSurfaceLineUpdateDataStrategyTest.cs
===================================================================
diff -u -ra8d5814e2cb003b85852c499cbc25ca0156887d4 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/RingtoetsPipingSurfaceLineUpdateDataStrategyTest.cs	(.../RingtoetsPipingSurfaceLineUpdateDataStrategyTest.cs)	(revision a8d5814e2cb003b85852c499cbc25ca0156887d4)
+++ Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/RingtoetsPipingSurfaceLineUpdateDataStrategyTest.cs	(.../RingtoetsPipingSurfaceLineUpdateDataStrategyTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -300,8 +300,7 @@
             // Assert
             var exception = Assert.Throws<UpdateDataException>(call);
 
-            string expectedMessage = "Profielschematisaties moeten een unieke naam hebben. " +
-                                     $"Gevonden dubbele elementen: {duplicateName}.";
+            const string expectedMessage = "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(expectedMessage, exception.Message);
 
             CollectionAssert.IsEmpty(targetCollection);
Index: Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/StochasticSoilModelUpdateDataStrategyTest.cs
===================================================================
diff -u -r50597fbeb76b752bc250107dfa91b696bb6c4995 -rb7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1
--- Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/StochasticSoilModelUpdateDataStrategyTest.cs	(.../StochasticSoilModelUpdateDataStrategyTest.cs)	(revision 50597fbeb76b752bc250107dfa91b696bb6c4995)
+++ Ringtoets/Piping/test/Ringtoets.Piping.Plugin.Test/FileImporter/StochasticSoilModelUpdateDataStrategyTest.cs	(.../StochasticSoilModelUpdateDataStrategyTest.cs)	(revision b7f91e5d6aaac3c9a52cfda9b1bdd3a74ee4f8b1)
@@ -152,8 +152,7 @@
 
             // Assert
             var exception = Assert.Throws<UpdateDataException>(test);
-            const string expectedMessage = "Stochastische ondergrondmodellen moeten een unieke naam hebben. " +
-                                           "Gevonden dubbele elementen: non-unique name.";
+            const string expectedMessage = "Geïmporteerde data moet unieke elementen bevatten.";
             Assert.AreEqual(expectedMessage, exception.Message);
 
             CollectionAssert.IsEmpty(targetCollection);