There's a difference between imitation and just having the same name.
###Unintentional coincidence
Example: 2 people named John who create an account with the same display name. The profile picture will most likely be different.
This is the case when two users just happen to use the same name, and is completely OK.
###Intentional impersonation
Example: 10 blade19899's with the exact same name and profile picture.
This is not allowed and the imitators should be flagged with an appropriate message in "other (needs ♦ moderator attention)".
This is supported by the SE Content Policy:
If you are found to be in violation of any of the below policies, you will receive a notice via email. Unless you explain or correct your behavior within 72 hours, your account will be suspended. We will do our best to work with you and ensure a fair outcome in all cases. We reserve the right to immediately suspend, without notice, any content, account, or IP address which we determine to be submitting spam or potentially other damaging or disruptive content the Network.
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Identity Theft and Privacy. Users that misleadingly appropriate the identity of another person are not permitted. [ ... ]