Recipients receives email double

Dear all,

Right now we have two clients, lotus notes client and outlook client.

Recently one of outlook user complain that he got emails, not all emails, double from only one sender that uses lotus notes.

If i check the double emails:

-one email in html format

-and one email in text only format

but, this is intermittent, not all email that he receives double.

in lotus notes preference, internet mail, we all use html and plain text format.

Any advise?

Best Regards