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I've just found out that when I CC people in emails they receive the message twice - I imagine that this is quite annoying for them. Anybody know why this happens and a sure fire solution?

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Are you sure this isn't an issue with either the sending or receiving mail servers? Have the recipient check the time stamp on the emails, if the duplicate emails have different time stamps, then it is not thunderbird. – MaQleod Dec 3 '10 at 17:22
(Given the "again" in your original title: if you failed to login using another existing account, then you might want to flag your question for moderator help?) – Arjan Dec 3 '10 at 19:40
You can try to CC a message to and then on use the "(text view)" link to see the headers. – Arjan Dec 3 '10 at 19:43

There's a good chance that their e-mail address is on the list twice.

Without seeing a header of the offending e-mails it's impossible to say for sure.

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