diff --git a/partitionmanager/table_append_partition.py b/partitionmanager/table_append_partition.py
index 44fb662..ed9687c 100644
--- a/partitionmanager/table_append_partition.py
+++ b/partitionmanager/table_append_partition.py
@@ -608,10 +608,12 @@ def _plan_partition_changes(
         )
 
     # Confirm we won't make timestamp conflicts
-    existing_timestamps = list(map(lambda p: p.timestamp(), partition_list))
     conflict_found = True
     while conflict_found:
         conflict_found = False
+
+        existing_timestamps = set(map(lambda p: p.timestamp(), partition_list))
+
         for partition in results:
             if partition.timestamp() in existing_timestamps:
                 if (
@@ -628,6 +630,8 @@ def _plan_partition_changes(
                 conflict_found = True
                 break
 
+            existing_timestamps.add(partition.timestamp())
+
     # Final result is always MAXVALUE
     results[-1].set_as_max_value()
 
diff --git a/partitionmanager/table_append_partition_test.py b/partitionmanager/table_append_partition_test.py
index 4084c39..45cb6d9 100644
--- a/partitionmanager/table_append_partition_test.py
+++ b/partitionmanager/table_append_partition_test.py
@@ -801,6 +801,50 @@ def test_plan_partition_changes_misprediction(self):
             ],
         )
 
+    def test_plan_partition_changes_misprediction_duplicate(self):
+        """We have to handle the case where a mispredicted rate of change
+        calculation produces results that themselves have duplicates"""
+        self.maxDiff = None
+        planned = _plan_partition_changes(
+            MockDatabase(),
+            Table("table"),
+            [
+                mkPPart("p_20220419", 81567449545, 99982222560),
+                mkPPart("p_20220519", 90007334722, 110234961540),
+                mkPPart("p_20220520", 94841841817, 116162938085),
+                mkPPart("p_20220521", 99676348912, 122090914630),
+                mkPPart("p_20220522", 102672012866, 127123677707),
+                mkTailPart("p_20220523", count=2),
+            ],
+            mkPos(90408556246, 110749398176),
+            datetime(2022, 5, 20, 18, 55, 16, 155, tzinfo=timezone.utc),
+            timedelta(days=30),
+            3,
+        )
+
+        # this configuration could prompt a duplicate p_20220524 partition, which
+        # should end up with the second being moved to 5-25
+
+        self.assertEqual(
+            planned,
+            [
+                ChangePlannedPartition(
+                    mkPPart("p_20220520", 94841841817, 116162938085)
+                ),
+                ChangePlannedPartition(mkPPart("p_20220521", 99676348912, 122090914630))
+                .set_timestamp(datetime(2022, 5, 24, tzinfo=timezone.utc))
+                .set_important(),
+                ChangePlannedPartition(
+                    mkPPart("p_20220522", 102672012866, 127123677707)
+                )
+                .set_timestamp(datetime(2022, 5, 25, tzinfo=timezone.utc))
+                .set_important(),
+                ChangePlannedPartition(mkTailPart("p_20220523", count=2)).set_timestamp(
+                    datetime(2022, 5, 26, tzinfo=timezone.utc)
+                ),
+            ],
+        )
+
     def test_get_rate_partitions_with_implicit_timestamps(self):
         eval_time = datetime(2021, 6, 8, tzinfo=timezone.utc)