When an account with the same sAMAcountName is created on multiple servers. We'll end with two account with the same name resulting in NT_STATUS_INTERNAL_DB_CORRUPTION on logon. This can happen when a member server runs net ads join twice.
This explains that one sAMAccountName should be renamed to $duplicate-<RID> http://social.technet.microsoft.com/wiki/contents/articles/15435.active-directory-duplicate-object-name-resolution.aspx#When_a_Duplicate_sAMAccountName_in_Domain_is_Detected