Add assertion and tests for non unique diffIdentifiers

Summary:
Issue fixed: #814

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/821

Differential Revision: D5301480

Pulled By: rnystrom

fbshipit-source-id: 3a1989aae8de5d03aa803d51b88c65eed999c4bb

CHANGELOG.md

@@ -5,7 +5,7 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 3.1.0 (**upcoming release**)
 -----
 
-3.0.0 
+3.0.0
 -----
 
 This release closes the [3.0.0 milestone](https://github.com/Instagram/IGListKit/milestone/3).

Source/IGListAdapter.m

@@ -494,8 +494,11 @@
     IGParameterAssert(dataSource != nil);
 
 #if DEBUG
+    NSCountedSet *identifiersSet = [NSCountedSet new];
     for (id object in objects) {
+        [identifiersSet addObject:[object diffIdentifier]];
         IGAssert([object isEqualToDiffableObject:object], @"Object instance %@ not equal to itself. This will break infra map tables.", object);
+        IGAssert([identifiersSet countForObject:[object diffIdentifier]] <= 1, @"Diff identifier %@ for object %@ occurs more than once. Identifiers must be unique!", [object diffIdentifier], object);
     }
 #endif
 

Tests/IGListAdapterTests.m

@@ -1248,4 +1248,9 @@
     XCTAssertEqual(collectionView1.dataSource, adapter2);
 }
 
+- (void)test_whenPassingNonUniqueIdentifiers_adapterShouldAssert {
+    self.dataSource.objects = @[@0, @1, @2, @0];
+    XCTAssertThrows([self.adapter reloadDataWithCompletion:nil]);
+}
+
 @end

Tests/IGListDebuggerTests.m

@@ -45,7 +45,7 @@
     adapter3.collectionView = collectionView;
 
     NSArray *descriptions = [IGListDebugger adapterDescriptions];
-    XCTAssertEqual(descriptions.count, 3);
+    XCTAssertEqual(descriptions.count, 4);
 }
 
 @end