diff --git a/Source/IGListAdapterUpdater.m b/Source/IGListAdapterUpdater.m
index 37c2be79..6166ca2f 100644
--- a/Source/IGListAdapterUpdater.m
+++ b/Source/IGListAdapterUpdater.m
@@ -132,6 +132,8 @@
         return;
     }
 
+    IGAssert(objectsWithDuplicateIdentifiersRemoved(fromObjects).count == fromObjects.count, @"The fromObjects has duplicate identifiers (it should already be dedupepd at this point).");
+    
     NSArray *toObjects = nil;
     if (toObjectsBlock != nil) {
         toObjects = objectsWithDuplicateIdentifiersRemoved(toObjectsBlock());