Is a unique username a must?

My customer doesn't want the username to be unique, but the "username+password" to be unique. At first I thought NO WAY, but I wanted to check with you guys first.
Right now I have the username unique, but I rely on a userID when handling users. This way the users can change their username if they wish so.

Now, I consider it bad practice to NOT have unique usernames, but should I submit to their will here?
Has anyone run in to any problems with non-unique usernames?
Will it make other parts of the implementation harder?
I don't want to implement it and then run in to a stone wall like "Damn it, this is not possible without unique usernames".

Any thoughts in general?


Thanks in advance!
/zalk

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