delete-duplicate-messages for IMAP folders is unsafe

Here is a happening:

Did vm-get-new-mail on an IMAP folder. The network connection was non-functional and VM hung. When the network connection was fixed, it ended with

   vm-imap-check-connection: IMAP error: "not connected"

When I did another vm-get-new-mail, there was another error:

   vm-update-draft-count: Format specifier doesn't match argument type

But this seems to have aborted the vm-get-new-mail for some reason. The new messages retrieved in the process presumably have nil fields for UID's.

The next try of vm-get-new-mail fetched new copies of the messages, which were fine, but vm-delete-duplicate-messages in the arrive-messaged-hook marked them for deletion.

Then VM says:

  Found 2 messages with invalid UIDs. Expunge them? (y or n)

The invalid UID's were for the first copies, and vm-delete-duplicate-messages has marked the new good copies for deletion. So, answering `y' at this point deletes all the copies.

This is clearly unsafe.