If any fields in the primary and secondary tables have the same name, Join automatically assigns a new name to the secondary field in the output table. For example, if both tables contain an Amount field, Join tries to name the secondary field Amount2, Amount3 and so on, until it finds a name that does not conflict with any field names in the output table.
Tip: In procedures and command mode, use the AS field modifier to specify unique names for your Join fields. For more information see As.