[Plugin: Simple Import Users] User name collision?

What happens when there is a collision in user names? Is there an option to automatically alter user names until they are unique?

I see Network is required. Are user names unique within a single subnetwork blog, or are they global across all network sites?

Incidentally, the description says email account generated where obviously user name generated is intended.

http://ww.wp.xz.cn/extend/plugins/simple-import-users/