Category exporting problem resulting in duplicate categories when importing

[catahac]

Certain chain of action lead to having duplicate categories when importing a wordpress export.
The steps to reproduce:

1. Create a Category but leave the slug to be auto-generated
2. Add a new post and attach it to that category
3. Change only the slug of the category
4. Export
5. Import
6. You will see that your category has been duplicated.

I just tested this in 2 fresh installs and the issue occurs

[scribu]

Reproduced in trunk.

Got two categories:

Test Cat | test-cat | 0 posts <-- this shouldn't be here
Test Cat | test-cat-bla | 1 posts

[janeforshort]

Reported too late to make it into 2.9. Punting due to lack of patch.

[scribu]

Marking as dup of #11354 since it has a patch already.