Deleting file from server creates duplicate nodes

I'm using node import 5.x-12 with Drupal 5.5. Everything goes OK until I hit the 'Delete CSV file from server' button as the last step (I'm importing a tsv file into a CCK content type). When I do that, all the nodes I imported are duplicated:

[Pre import]
mysql> select count(*) from node;
+----------+
| count(*) |
+----------+
| 274 |
+----------+
1 row in set (0.00 sec)

[Post 'Apply (import nodes)']
mysql> select count(*) from node;
+----------+
| count(*) |
+----------+
| 617 |
+----------+
1 row in set (0.00 sec)

[Post 'Delete CSV file from server']
mysql> select count(*) from node;
+----------+
| count(*) |
+----------+
| 960 |
+----------+
1 row in set (0.00 sec)

I can repeat this consistently.