Attaching files by dragging them to Mail icon creates two emails??

Same problem here. One new message window with the dragged attachment and one without.

The problem remains also with 10.5.2

Having the same problem - two new messages created when dropping a file on the dock icon. I can positively confirm that it is related to MailTags. Removing the MailTags bundle from ~/Library/Mail/Bundles and restarting Mail fixes the problem for me instantly. I have Mail ActOn and GrowlMail 1.1.3b2 bundles loaded as well. These do not cause the same problem.

Omarkow wrote:
The problem remains also with 10.5.2

Interesting, I was just going to post that 10.5.2 made it stop for me.

I have had my share of the problem too; not only did Mail open two windows when I dropped a file on its dock icon, it did so for each file (meaning two files would get me four windows). After reading the posts here, I tried removing all my Mail plug-ins, and then re-enabling one by one. As it turned out it solved the problem, but not because one of the plug-ins was the culprit, but because I quit and restarted Mail when I did. To my great surprise dropping one or more files on the dock icon works just fine now that all plug-ins are re-enabled and Mail has restarted, whether Mail is running, hidden, or or not. However, I found that using an Automator workflow with the create new mail action will immediately revert Mail's behaviour to the unwanted multi-window one. So… something is broken, but what?

Edit: I just tried the AppleScript posted above, and it has the same effect as using the Automator workflow. Obviously, the problem lies with Mail's AppleScript library.

another annoyed person with this problem. CAn't believe it's still not fixed

Tsetse... Although I am really a huge Apple fan, there we see that working with "black boxes" is not always an advantage. In the world of (Linux) OpenSource, this tiny - but annoying - bug would have been fixed the next day. But with Apple, we have to wait months and months, cause only they can do it for us... Grrr...

I have Default Folder X and it does not happen to me. Mail works as it should.

This is not related to any particular app. It's caused by whatever you might have in your ~/Library/Mail/Bundles/
Try and drag out whatever you have in that "Bundles" folder and restart Mail.
That did it for me. I was using a bundle caled "MailUnreadMenu"