Duplicated emails received when sending to group and single user

Hello.

When someone sends a message to a group and also to a group member, message arrives twice to the group member. For example:
- There is a group mygroup@mysite.com
- Email adress myemail@anothersite.com is member of the group mygroup@mysite.com
- Someone sends a message to mygroup@mysite.com and myemail@anothersite.com
- Email adress myemail@anothersite.com receives two messages instead of one.

This only happens when user has an external address; if user is using an address of the domain (@mysite.com in the example), it works properly.

There is some way to avoid that behavior?

Thank you.