In rare cases, a Contact may have more than one User record. This situation may result from users who create a new User record with the New action from the Users page and assign the new User to a Contact that already has one. It may also happen when someone edits a User record and changes the Contact it's assigned to and selects a new Contact that already has a User record. 

More frequently, you may find that Contacts using the same email address. This usually happens for one of two reasons: 1) family members are sharing an email address (for example, a husband and wife had a shared email), or 2) a parent's email is also listed as their child's email address. When this happens, users will not be able to log in using the shared email address or use the shared email address to reset a password. To mitigate these concerns, you can:

• Encourage people to use a unique email address.
• If Users continue to share an email address, make sure they have a User Name that is not the shared email address.
• Note: If Users share an email address, they will not be able to use the Forgot Password option. (This might be the motivating "stick" when encouraging unique email address.)
  

Merging Duplicate User Records 

A nightly data quality routine looks for this scenario. When one is found, the Contact record is duplicated, the additional User record associated with the new record is created, and a "Duplicate of" relationship is added to tie both records together. You can then use the Combine Contacts tool to merge the duplicates.

Once completed, all User Groups and Security Roles are transferred to the remaining User record. If the User record has more than one instance of a Security Role, only one is kept.