Outlook 2003 sending messages twice

[malagash]

From time to time some of my recipients (and only some) notify me that they get everything I am sending twice. Other recipients only get a single copy. The emails always arrive within a minute of each other and they are always exact copies.

I am using Outlook 2003 and have server timeout set to one minute. setting it to a longer interval makes no difference

Anyone have any ideas as to why this happens?

Thanks