Duplicate messages from Plusnet - why?

[#1]

I joined Plusnet 2 weeks ago and the communication has been excellent in that time. There is however something that puzzles me and I hope someone will be able to enlighten me. Every time I receive a message from Plusnet, it is accompanied by a duplicate of that same message.  Why is that and how can I stop the duplication.

[#2]

One will be to postmaster@ (which is an official mailing address), the other to the address you've set for Contact details: Email in your Member Centre settings.
I find it convenient to direct the messages to different mailboxes. But, if you don't want two emails putting the postmaster address in Contact details will prevent duplication.

[#3]

Thanks for your reply. I will set up the contacts tomorrow.