Discussion list duplicates every post

I’ve sent in 2 support requests and have heard nothing so far.

My discussion list has decided to send out 2 copies of each post. It started happening suddenly (I hadn’t made any changes) and I can’t make it stop. Obviously, it’s quite annoying to have 2 copies of each message.

Has anyone else experienced this? Advice?

Thanks in advance - I’ll reply with results from any suggestions.