Apple Mail sent twice

Hello,

i have a MacBook 13" Late 2008 with the latest available OS X (El Capitan 10.11.6).

Sometimes Apple Mail sent an email twice. It's not a visual glitch in sent items folder. The recipiens told me that they receive two identical emails at the same time.

I reinstalled El Capitan once and started from scratch (no data was transfered from Time Machine). Didn't solved the problem.

The email server i use is a standard IMAP mailserver (no outlook junk). I have no such problems with other accounts and i'm not aware of other people having such a problem with their email accounts on the same server.