Index: Ringtoets/HydraRing/src/Ringtoets.HydraRing.Calculation/Services/HydraRingConfigurationService.cs
===================================================================
diff -u -r1c5621a4920e3d5821ccbf3b5d32a6eeb7658cba -r0280e7ce5a21162e00e3c092aff5a121777fd3ec
--- Ringtoets/HydraRing/src/Ringtoets.HydraRing.Calculation/Services/HydraRingConfigurationService.cs	(.../HydraRingConfigurationService.cs)	(revision 1c5621a4920e3d5821ccbf3b5d32a6eeb7658cba)
+++ Ringtoets/HydraRing/src/Ringtoets.HydraRing.Calculation/Services/HydraRingConfigurationService.cs	(.../HydraRingConfigurationService.cs)	(revision 0280e7ce5a21162e00e3c092aff5a121777fd3ec)
@@ -124,11 +124,11 @@
         /// the same <see cref="HydraRingSection.SectionId"/> has already been added.</exception>
         public void AddHydraRingCalculationInput(HydraRingCalculationInput hydraRingCalculationInput)
         {
-            var conflictingIds = hydraRingCalculationInputs.Where(h => h.Section.SectionId == hydraRingCalculationInput.Section.SectionId);
-            if (conflictingIds.Any())
+            if (hydraRingCalculationInputs.Any(h => h.Section.SectionId == hydraRingCalculationInput.Section.SectionId))
             {
                 throw new ArgumentException(@"Section id is not unique", "hydraRingCalculationInput");
             }
+
             hydraRingCalculationInputs.Add(hydraRingCalculationInput);
         }
 
Index: Ringtoets/HydraRing/test/Ringtoets.HydraRing.Calculation.Test/Services/HydraRingConfigurationServiceTest.cs
===================================================================
diff -u -r1c5621a4920e3d5821ccbf3b5d32a6eeb7658cba -r0280e7ce5a21162e00e3c092aff5a121777fd3ec
--- Ringtoets/HydraRing/test/Ringtoets.HydraRing.Calculation.Test/Services/HydraRingConfigurationServiceTest.cs	(.../HydraRingConfigurationServiceTest.cs)	(revision 1c5621a4920e3d5821ccbf3b5d32a6eeb7658cba)
+++ Ringtoets/HydraRing/test/Ringtoets.HydraRing.Calculation.Test/Services/HydraRingConfigurationServiceTest.cs	(.../HydraRingConfigurationServiceTest.cs)	(revision 0280e7ce5a21162e00e3c092aff5a121777fd3ec)
@@ -49,13 +49,13 @@
         {
             // Setup
             var hydraRingConfigurationService = new HydraRingConfigurationService("34-1", HydraRingTimeIntegrationSchemeType.NTI, HydraRingUncertaintiesType.Model);
-            var calculationInput = new HydraRingCalculationInputImplementation(1, 2);
+            var calculationInput1 = new HydraRingCalculationInputImplementation(1, 2);
+            var calculationInput2 = new HydraRingCalculationInputImplementation(1, 3);
 
-            // Precondition
-            hydraRingConfigurationService.AddHydraRingCalculationInput(calculationInput);
+            hydraRingConfigurationService.AddHydraRingCalculationInput(calculationInput1);
 
             // Call
-            TestDelegate test = () => hydraRingConfigurationService.AddHydraRingCalculationInput(calculationInput);
+            TestDelegate test = () => hydraRingConfigurationService.AddHydraRingCalculationInput(calculationInput2);
 
             // Assert
             TestHelper.AssertThrowsArgumentExceptionAndTestMessage<ArgumentException>(test, "Section id is not unique");