Read that email again. And again.

Same issue. Tried deleting "MessageUidsAlreadyDownloaded3" from Library/Mail, and also tried deleting "com.apple.mail.plist". Neither seems to have helped. I'm using POP mail. I deleted all the mail I had off the server, but now the issue is just repeating with new mail.

The really weird thing is that when I upgraded my old computer to 10.6, it didn't have this issue at all: it knows what not to download. It's only happening on my new computer, which to the naked eye has exactly the same Mail settings.

I guess it's not 10.6.1, because i have a powerbook g4 with leopard 10.5.8, an imac with 10.6.1' and an ipod touch, and this has been happenning with all three of them....

Didn't have any problems with dupe eMails after upgrading to 10.6. I've always used IMAP.

The only time it occurred was when I reinstalled the beta Snow Leopard compatible version of MailTags by InDev Software. Then I got all kinds of duplicate & disappearing eMails. I love the utility ... but it had to be banished until it's ready for prime time.

I've had this issue since 10.5 Leopard. Emails pop up as new even though they've been read, deleted and moved or flagged - it all just goes away. I'm using IMAP. At first thought it was my Nokia phone messy but now I'm using iPhone 3GS and OS X 10.6.1.

My service provider says Mail.app is leaving some IMAP stuff open and not syncing but instead making a new connection every time and not closing old one... or something like that. They said they have carefully checked that they are using IMAP standards and it is Mail.app that's acting wrong.

I do have this problem only with my own domain. University email and Gmail work just fine.

I had the same problem and found a fix on the apple boards. I'm not sitting in front of my home laptop right now, but it was deleting the "messagesUIDsalreadydownloaded" (or something like that) file from the ~/Library/Mail. I also deleted that same file from each individual account. That seems to have worked for the last week and a half or so. *Knock on wood*

Someone else do it and see if it's the placebo effect or not

I received a double-message today, but I'm guessing it was accidentally sent out twice, since I'm still using Leopard, and it was the only duplicated message out of the fifteen I had received.

What bothers me, though, is that when I send a message, the status in the bottom-left almost always says it's sending two messages out.

10.6.1 gives me zombies in my RSS feeds. Deleted messages come back from the dead when mail is restarted.

I now have the same problem on my 10.6.1 Mac as well. Both the 10.5.8.and the 10.6.1 machines have different Mobile Me accounts (IMAP) and share the same issue starting today. This is not a problem directly related to Snow Leopard.

I have the problem as well under OS X 10.5.8, Mail version 3.6. Mobile Me account (IMAP).

i gave up on mail when I moved to 10.4, they had it right in 10.3 but since 4 they lost it. I went to T-Bird and have not looked back since.