Apparently user names don't have to be unique, see the comments to this answer. Was this a good idea? In the comments I mentioned it's only clear which Mark is which because one is the OP and hence has his name formatted differently. In other situations it may be harder to tell them apart.
Shouldn't we for future registrations stick to unique user names?
This issue has been brought up in various forms a decent amount on meta.StackOverflow and probably wont change by a question here. See the meta post "Why does Stack Overflow allow duplicate display names?"
Basically it looks like they want it to be "Real Life" and also to not favor users who have been around longer. At least that is what the answers to that post say and they were never rebutted.
Seems reasonable to me, but then again I don't have to implement it.
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\$\begingroup\$ Shouldn't be too difficult me thinks, just look up the new user name when registering. It's not that every page should get extra scripting. \$\endgroup\$ – stevenvh Jul 1 '11 at 12:48
mark
hoping it will be unique. \$\endgroup\$ – stevenvh Jul 10 '11 at 1:58