diff --git a/resources/sql/autopatches/20170918.ref.01.position.php b/resources/sql/autopatches/20170918.ref.01.position.php
new file mode 100644
--- /dev/null
+++ b/resources/sql/autopatches/20170918.ref.01.position.php
@@ -0,0 +1,52 @@
+<?php
+
+$table = new PhabricatorRepositoryRefPosition();
+$conn = $table->establishConnection('w');
+$key_name = 'key_position';
+
+try {
+  queryfx(
+    $conn,
+    'ALTER TABLE %T DROP KEY %T',
+    $table->getTableName(),
+    $key_name);
+} catch (AphrontQueryException $ex) {
+  // This key may or may not exist, depending on exactly when the install
+  // ran previous migrations and adjustments. We're just dropping it if it
+  // does exist.
+
+  // We're doing this first (outside of the lock) because the MySQL
+  // documentation says "if you ALTER TABLE a locked table, it may become
+  // unlocked".
+}
+
+queryfx(
+  $conn,
+  'LOCK TABLES %T WRITE',
+  $table->getTableName());
+
+$seen = array();
+foreach (new LiskMigrationIterator($table) as $position) {
+  $cursor_id = $position->getCursorID();
+  $hash = $position->getCommitIdentifier();
+
+  // If this is the first copy of this row we've seen, mark it as seen and
+  // move on.
+  if (empty($seen[$cursor_id][$hash])) {
+    $seen[$cursor_id][$hash] = true;
+    continue;
+  }
+
+  // Otherwise, get rid of this row as it duplicates a row we saw previously.
+  $position->delete();
+}
+
+queryfx(
+  $conn,
+  'ALTER TABLE %T ADD UNIQUE KEY %T (cursorID, commitIdentifier)',
+  $table->getTableName(),
+  $key_name);
+
+queryfx(
+  $conn,
+  'UNLOCK TABLES');