For various reasons, there are times when Active Directory may encounter a situation where multiple objects appear. The duplicates will be named with a CNF:[GUID] on the end of the name.
Here is an excellent Microsoft blog post with an explanation of why it happens and some simple commands to find these objects. It is an issue we encounter at times with Configuration Manager boundaries and slow AD replication, but could happen for many other object types also.
For a slightly simpler method, here is my approach to using AD Users and Computers to find and delete them.