On your BlackBerry, does your inbox get a copy of messages you send?

This behavior sounds exactly like what happens when your BlackBerry is
set up to access Gmail using its built-in mail client. It all has to
do with POP -- which is how Gmail's servers are being accessed by the
BlackBerry's built-in client.

So what can you do about it? Here are two options:

1. Create a filter. You can create a filter in your BlackBerry
Internet service account. For instructions on how to create a filter,
visit http://mail.google.com/support/bin/answer.py?answer=16418 and
click the 'BlackBerry Technical Solution Center' link.

2. Use the app. Download and install the Gmail for mobile application
at http://m.google.com/mail.

3. Use the web. Use your BlackBerry's web browser to access http://gmail.com.

These options might not suit all users, but feel free to give them a
try and see which one works best for you!