Did not happen to me and I've done it a ton of times both in a local environment and on the real server.
The important thing was a reinstall (did not have to reupload files) and clearing out the old xF data.
If you don't reinstall, you'll get duplicate. Likewise, if you run the importer and don't complete it, and then choose to discontinue and redo the import, you'll also get duplicates. However, it does warn you that it happens.
Deleting the install-lock and reinstalling always worked for me.