I'm sorry, the phpBB search quite frankly sucks. I will see if I can hack in a vBulletin-esque (higher quality results) search function.
I have tested and verified your report in the most recent ListMail files. It seems both confirmations are coming with the same UID, which might not be right depending on what features (or bugs) I've added recently.
I'm having trouble recreating this. With further tests I'm getting different UIDs/confirm links for each user.
Unfortunately, I am going to have to look into this further in the morning. I apologize for the delay - I was very busy today.
Regards