Sent box full of messages I haven’t sent

My sent box keeps filling up with messages as if I've sent to myself - pages and pages of them.  I delete them and it does it all over again the next day.  They all seem to be duplication's of emails that I have sent previously....I can't understand it?  Can anyone please help?  Only recently created this new email address - don't want to have to delete and start again if poss as have sent all my contacts my new email address.  many thanks in advance. Loz xxx