Mail forwarding

[pch0317]

Hi,
I want to forward mail only for one user u...@mydomain.tld to 
u...@otherdomain.tld.

I try do it in this way:
/etc/postfix/virtual:
  ...
  u...@mydomain.tld u...@mydomain.tld, u...@otherdomain.tld
  ...

When I send mail to u...@mydomain.tld it receive message but user 
u...@otherdomain.tld receive 2 identical copy of mail.
How to send only one copy of mail to u...@otherdomain.tld


Thanks