When a user logs in, if there is a contact record in your database that has the same email address associated with it, the user will be connected to that contact record. (This does not work if you turn off email verification.)
Scenarios/Examples
#1 - Mom creates a contact record and add's her husband's work email as his contact email address. Dad logs in later and uses his personal email address. Since that email address is not in the system, a new contact record is created. To clean up - move Dad's preferred email address to contact record #1 for the parent 2 email address and then cancel the second contact record.
#2 - You uploaded or transferred a list of contacts in to your database. When users log in they choose to use a different email address. There is no value in keeping the first contact record. Cancel that record.
We have a new report that will allow administrators to find duplicate contact records.