I'm developing a network of sites based on one WordPress

I'm developing a network of sites based on one WordPress network.

I have BP Multi Network set up and working correctly and I have unique BuddyPress networks working on individual sites.

My question is this...

As all of these sites use the same WordPress database, is there any way to get around the issue of username duplication?

If someone on site A has taken the username 'johnsmith' but some on one site b would like to be 'johnsmith' too the user name will be unavailable. Is this correct? Is there any way around it?