Long shot, but are the two copies identical? If one is a forwarding copy of the other, it could be caused by a message filter configured to forward messages not working as/when expected…

If this is a gmail or outlook.com/hotmail.com account, they both copy sent mail automatically, so 'Place a copy in' in Account Settings, Copies & Folders, should be unchecked.

I was wondering that… How does he know that he's really sending the same message twice and not just having the sent message being saved twice?

Thanks to all for responding. The messages are identical. I had the Copy To checked. Removing this to see if it works. I think DavidGG may be right and I was getting two in the sent file.

It would be sfhowes who was right. He correctly figured you were misunderstanding what was happening. I made the mistake of thinking your description of the problem was correct and only after having replied started to consider the possibility that it wasn't… :)