Username questions? 10 points!?

[DunbarPappy®ϟϟ]

Usernames can be nearly identical to another without trouble in the technical sense, since the data is digital and specific to the characters used, but again, it might matter how much you want associated with other sources, or if you intend to be agnostic towards another user.

The only time you'd want to have the exact same username (those that are public viewable) would be if you want to have an association between several accounts, and/or several sites.
Having the same username could be problematic if someone were to get an attitude, and begin harassing you...and using all available websites to that end could be a major pain.
This would apply to stalking also. By associating several account locations and profiles, divulged information (no matter how trivial), revealing friends (and subsequently investigating their accounts, who in turn reveal things unknown to you), and other tid-bits scattered across the 'Net, could lead to serious trouble.

Privacy is getting harder to attain on the 'Net and users should make every effort to guard against intrusions of any & all kinds.
Websites are notorious for making sudden releases of account information without informed consent, and that's another reason to remain anon as best you can.
Facebook is famous for this tactic.

Just keep a notebook with all your personas and the associated website, usernames, passwords, and so on, there. That keeps you up with your activity.