Trouble re-importing from .xml file

I imported posts from a .xml file. Fine.
I deleted them all.

Now, when I try to re-import again, I get a list of every post saying:
Post “post title” already exists.

But, they don’t exist. They aren’t in there anywhere.

How can I get it to reset and allow the new import?

Also, I tried re-exporting from the original source. No help.