Win 7 Login Requests Password for Wrong User Account

Over the week-end something strange happened to one of our Windows 7 systems. It worked fine on Friday, was shut down for the week-end. When turned on this morning everything appeared normal. There are six user accounts set up on the system. No matter which one you choose to log in to, it requests the password for the first user set up on the system. When the screen comes up requesting the password, you can click on Switch User and then when you choose a user to log in as, it prompts for the correct user's password. I have never seen this happen and am at a loss as to how to correct.

Any suggestions would be greatly appreciated! Thanks in advance.

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