phpBB2Privatemsg creates duplicate entries

When importing PMs from a phpBB 2.23 database, some PMs (those that get the first message IDs) are listed in many places due to false entries in the pm_index table.

The former code counts matching entries and remaps that count wrongly onto a message-ID.
Then it proceedes with inserting these wrong message-IDs twice into the pm_index table, anyway.
This results in PM-threads prepended with PM number 1, 2 and sometimes 3 -- that probably don't even belong neither to the sender, nor the receiver of the regular PM that follows.

The fixed code simply rejects to further do anything with double PMs.
This assumes that phpBB-2.x-PMs are on a one-to-one basis and can not be sent to a list of users.

Thanks for the module!