Index: Core/Common/src/Core.Common.Utils/Extensions/EnumerableExtensions.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Core/Common/src/Core.Common.Utils/Extensions/EnumerableExtensions.cs	(.../EnumerableExtensions.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Core/Common/src/Core.Common.Utils/Extensions/EnumerableExtensions.cs	(.../EnumerableExtensions.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -77,5 +77,28 @@
             }
             return source.Select(keySelector).Count() != source.Select(keySelector).Distinct().Count();
         }
+
+        /// <summary>
+        /// Checks whether the elements from <paramref name="source"/> have more than one unique 
+        /// value using the <paramref name="keySelector"/>.
+        /// </summary>
+        /// <typeparam name="T">The element type of the sequence.</typeparam>
+        /// <typeparam name="TKey">The type for the key.</typeparam>
+        /// <param name="source">A sequence that contains elements to be acted upon.</param>
+        /// <param name="keySelector">The key selector to validate uniqueness.</param>
+        /// <returns><c>true</c> when there is more than one unique value in <see cref="source"/>; <c>false</c> otherwise.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when any input argument is <c>null</c>.</exception>
+        public static bool HasMultipleUniqueValues<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector)
+        {
+            if (source == null)
+            {
+                throw new ArgumentNullException(nameof(source));
+            }
+            if (keySelector == null)
+            {
+                throw new ArgumentNullException(nameof(keySelector));
+            }
+            return source.Select(keySelector).Distinct().Count() > 1;
+        }
     }
 }
\ No newline at end of file
Index: Core/Common/test/Core.Common.Utils.Test/Extensions/EnumerableExtensionsTest.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Core/Common/test/Core.Common.Utils.Test/Extensions/EnumerableExtensionsTest.cs	(.../EnumerableExtensionsTest.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Core/Common/test/Core.Common.Utils.Test/Extensions/EnumerableExtensionsTest.cs	(.../EnumerableExtensionsTest.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -142,5 +142,69 @@
             // Assert
             Assert.IsTrue(hasNonDistinct);
         }
+
+        [Test]
+        public void HasMultipleUniqueValues_IteratorNull_ThrowArgumentNullException()
+        {
+            // Setup
+            IEnumerable<object> enumerable = null;
+
+            // Call
+            TestDelegate call = () => enumerable.HasMultipleUniqueValues<object, object>(e => null);
+
+            // Assert
+            var exception = Assert.Throws<ArgumentNullException>(call);
+            Assert.AreEqual("source", exception.ParamName);
+        }
+
+        [Test]
+        public void HasMultipleUniqueValues_ActionNull_ThrowArgumentNullException()
+        {
+            // Setup
+            IEnumerable<object> enumerable = Enumerable.Empty<object>();
+
+            // Call
+            TestDelegate call = () => enumerable.HasMultipleUniqueValues<object, object>(null);
+
+            // Assert
+            var exception = Assert.Throws<ArgumentNullException>(call);
+            Assert.AreEqual("keySelector", exception.ParamName);
+        }
+
+        [Test]
+        public void HasMultipleUniqueValues_WithUniqueItems_ReturnsFalse()
+        {
+            // Setup
+            var items = new List<int>
+            {
+                1,
+                1,
+                1
+            };
+
+            // Call
+            bool hasNonDistinct = items.HasMultipleUniqueValues(t => t);
+
+            // Assert
+            Assert.IsFalse(hasNonDistinct);
+        }
+
+        [Test]
+        public void HasMultipleUniqueValues_WithMultipleUniqueItems_ReturnsTrue()
+        {
+            // Setup
+            var items = new List<int>
+            {
+                1,
+                2,
+                3
+            };
+
+            // Call
+            bool hasNonDistinct = items.HasMultipleUniqueValues(t => t);
+
+            // Assert
+            Assert.IsTrue(hasNonDistinct);
+        }
     }
 }
\ No newline at end of file
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Factories/RingtoetsStackChartDataFactory.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Factories/RingtoetsStackChartDataFactory.cs	(.../RingtoetsStackChartDataFactory.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Factories/RingtoetsStackChartDataFactory.cs	(.../RingtoetsStackChartDataFactory.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -26,7 +26,6 @@
 using Core.Common.Base.Data;
 using Core.Common.Utils.Extensions;
 using Core.Components.Stack.Data;
-using Ringtoets.Common.Forms.Helpers;
 using Ringtoets.Common.Forms.Properties;
 using Ringtoets.Common.Forms.Views;
 
@@ -70,7 +69,7 @@
                 illustrationPointControlItems.Select(controlItem => Tuple.Create(controlItem.WindDirectionName,
                                                                                  controlItem.ClosingSituation));
 
-            bool showClosingSituation = !illustrationPointControlItems.HasDuplicates(item => item.ClosingSituation);
+            bool showClosingSituation = illustrationPointControlItems.HasMultipleUniqueValues(item => item.ClosingSituation);
 
             foreach (Tuple<string, string> label in labels)
             {
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/PropertyClasses/StructuresOutputProperties.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/Common/src/Ringtoets.Common.Forms/PropertyClasses/StructuresOutputProperties.cs	(.../StructuresOutputProperties.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/PropertyClasses/StructuresOutputProperties.cs	(.../StructuresOutputProperties.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -109,9 +109,9 @@
                     return new TopLevelFaultTreeIllustrationPointProperties[0];
                 }
 
-                bool areClosingSituationsSame = data.GeneralResult
-                                                    .TopLevelIllustrationPoints
-                                                    .HasDuplicates(p => p.ClosingSituation);
+                bool areClosingSituationsSame = !data.GeneralResult
+                                                     .TopLevelIllustrationPoints
+                                                     .HasMultipleUniqueValues(p => p.ClosingSituation);
 
                 return data.GeneralResult
                            .TopLevelIllustrationPoints
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Views/GeneralResultFaultTreeIllustrationPointView.cs
===================================================================
diff -u -r68ad4c0477e2ae7f325fef005b88d5be5c563840 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Views/GeneralResultFaultTreeIllustrationPointView.cs	(.../GeneralResultFaultTreeIllustrationPointView.cs)	(revision 68ad4c0477e2ae7f325fef005b88d5be5c563840)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Views/GeneralResultFaultTreeIllustrationPointView.cs	(.../GeneralResultFaultTreeIllustrationPointView.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -127,9 +127,9 @@
                 return;
             }
 
-            string closingSituation = generalResult.TopLevelIllustrationPoints.HasDuplicates(p => p.ClosingSituation)
-                                          ? string.Empty
-                                          : topLevelFaultTreeIllustrationPoint.ClosingSituation;
+            string closingSituation = generalResult.TopLevelIllustrationPoints.HasMultipleUniqueValues(p => p.ClosingSituation)
+                                          ? topLevelFaultTreeIllustrationPoint.ClosingSituation
+                                          : string.Empty;
 
             var faultTreeIllustrationPoint = selection.Data as FaultTreeIllustrationPoint;
             if (faultTreeIllustrationPoint != null)
Index: Ringtoets/Common/src/Ringtoets.Common.Forms/Views/IllustrationPointsTableControl.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/Common/src/Ringtoets.Common.Forms/Views/IllustrationPointsTableControl.cs	(.../IllustrationPointsTableControl.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/Common/src/Ringtoets.Common.Forms/Views/IllustrationPointsTableControl.cs	(.../IllustrationPointsTableControl.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -94,7 +94,7 @@
         {
             if (data != null)
             {
-                if (!data.HasDuplicates(item => item.ClosingSituation))
+                if (data.HasMultipleUniqueValues(item => item.ClosingSituation))
                 {
                     illustrationPointsDataGridViewControl.SetColumnVisibility(closingSituationColumnIndex, true);
                     return;
Index: Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/DikeHeightOutputProperties.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/DikeHeightOutputProperties.cs	(.../DikeHeightOutputProperties.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/DikeHeightOutputProperties.cs	(.../DikeHeightOutputProperties.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -189,9 +189,9 @@
                     return new TopLevelFaultTreeIllustrationPointProperties[0];
                 }
 
-                bool areClosingSituationsSame = data.GeneralResult
-                                                    .TopLevelIllustrationPoints
-                                                    .HasDuplicates(p => p.ClosingSituation);
+                bool areClosingSituationsSame = !data.GeneralResult
+                                                     .TopLevelIllustrationPoints
+                                                     .HasMultipleUniqueValues(p => p.ClosingSituation);
 
                 return data.GeneralResult
                            .TopLevelIllustrationPoints
Index: Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingOutputProperties.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingOutputProperties.cs	(.../OvertoppingOutputProperties.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingOutputProperties.cs	(.../OvertoppingOutputProperties.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -196,9 +196,9 @@
                     return new TopLevelFaultTreeIllustrationPointProperties[0];
                 }
 
-                bool areClosingSituationsSame = data.GeneralResult
-                                                    .TopLevelIllustrationPoints
-                                                    .HasDuplicates(p => p.ClosingSituation);
+                bool areClosingSituationsSame = !data.GeneralResult
+                                                     .TopLevelIllustrationPoints
+                                                     .HasMultipleUniqueValues(p => p.ClosingSituation);
 
                 return data.GeneralResult
                            .TopLevelIllustrationPoints
Index: Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingRateOutputProperties.cs
===================================================================
diff -u -r2e944cbf52d124fd501ed4ed8e991aa50299cc69 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingRateOutputProperties.cs	(.../OvertoppingRateOutputProperties.cs)	(revision 2e944cbf52d124fd501ed4ed8e991aa50299cc69)
+++ Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Forms/PropertyClasses/OvertoppingRateOutputProperties.cs	(.../OvertoppingRateOutputProperties.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -189,9 +189,9 @@
                     return new TopLevelFaultTreeIllustrationPointProperties[0];
                 }
 
-                bool areClosingSituationsSame = data.GeneralResult
-                                                    .TopLevelIllustrationPoints
-                                                    .HasDuplicates(p => p.ClosingSituation);
+                bool areClosingSituationsSame = !data.GeneralResult
+                                                     .TopLevelIllustrationPoints
+                                                     .HasMultipleUniqueValues(p => p.ClosingSituation);
 
                 return data.GeneralResult
                            .TopLevelIllustrationPoints
Index: Ringtoets/Integration/src/Ringtoets.Integration.Plugin/RingtoetsPlugin.cs
===================================================================
diff -u -ra49798da6ac2176d2e8b28ec6fec8a2494d8ee91 -ra163c5f082694baf5cd491ea36baf6fb040fe868
--- Ringtoets/Integration/src/Ringtoets.Integration.Plugin/RingtoetsPlugin.cs	(.../RingtoetsPlugin.cs)	(revision a49798da6ac2176d2e8b28ec6fec8a2494d8ee91)
+++ Ringtoets/Integration/src/Ringtoets.Integration.Plugin/RingtoetsPlugin.cs	(.../RingtoetsPlugin.cs)	(revision a163c5f082694baf5cd491ea36baf6fb040fe868)
@@ -48,6 +48,7 @@
 using Ringtoets.Common.Data.DikeProfiles;
 using Ringtoets.Common.Data.FailureMechanism;
 using Ringtoets.Common.Data.Hydraulics;
+using Ringtoets.Common.Data.IllustrationPoints;
 using Ringtoets.Common.Data.Structures;
 using Ringtoets.Common.Forms.ChangeHandlers;
 using Ringtoets.Common.Forms.GuiServices;
@@ -110,7 +111,6 @@
 using RingtoetsIntegrationPluginResources = Ringtoets.Integration.Plugin.Properties.Resources;
 using BaseResources = Core.Common.Base.Properties.Resources;
 using GuiResources = Core.Common.Gui.Properties.Resources;
-using Ringtoets.Common.Data.IllustrationPoints;
 
 namespace Ringtoets.Integration.Plugin
 {
@@ -844,6 +844,16 @@
             }
         }
 
+        #region PropertyInfos
+
+        private static TopLevelFaultTreeIllustrationPointProperties CreateTopLevelFaultTreeIllustrationPointProperties(SelectedTopLevelFaultTreeIllustrationPoint point)
+        {
+            return new TopLevelFaultTreeIllustrationPointProperties(point.TopLevelFaultTreeIllustrationPoint,
+                                                                    !point.ClosingSituations.HasMultipleUniqueValues(cs => cs));
+        }
+
+        #endregion
+
         private class FailureMechanismContextAssociation
         {
             private readonly Func<IFailureMechanism, IAssessmentSection, object> createFailureMechanismContext;
@@ -866,16 +876,6 @@
             }
         }
 
-        #region PropertyInfos
-
-        private static TopLevelFaultTreeIllustrationPointProperties CreateTopLevelFaultTreeIllustrationPointProperties(SelectedTopLevelFaultTreeIllustrationPoint point)
-        {
-            return new TopLevelFaultTreeIllustrationPointProperties(point.TopLevelFaultTreeIllustrationPoint,
-                                                                    point.ClosingSituations.HasDuplicates(cs => cs));
-        }
-
-        #endregion
-
         #region ViewInfos
 
         #region FailureMechanismView ViewInfo