Improve FrozenDictionary.ToFrozenDictionary for Dictionary sources with different comparers

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs

@@ -129,7 +129,7 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
             // Ensure we have a Dictionary<,> using the specified comparer such that all keys
             // are non-null and unique according to that comparer.
             newDictionary = source as Dictionary<TKey, TValue>;
-            if (newDictionary is null || (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer)))
+            if (newDictionary is null)
             {
                 newDictionary = new Dictionary<TKey, TValue>(comparer);
                 foreach (KeyValuePair<TKey, TValue> pair in source)
@@ -140,6 +140,18 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
                     newDictionary[pair.Key] = pair.Value;
                 }
             }
+            else if (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer))
+            {
+                var dictionary = new Dictionary<TKey, TValue>(comparer);
+                foreach (KeyValuePair<TKey, TValue> pair in newDictionary)
+                {
+                    // Dictionary's constructor uses Add, which will throw on duplicates.
+                    // This implementation uses the indexer to avoid throwing and to overwrite
+                    // existing entries such that last one wins.
+                    dictionary[pair.Key] = pair.Value;
+                }
+                newDictionary = dictionary;
+            }
 
             if (newDictionary.Count == 0)
             {