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Using the mass customer change report, the system can

You cannot replace a customer master record that already exists for a partner role with another customer master record.

In Customizing, you can specify that certain partner roles must be unique in the customer master record. If this type of partner role already exists in the customer master record, the system cannot make a second entry with the same role.
If you have flagged certain roles as mandatory partner functions, you cannot remove this entry from the customer master record using the mass customer change.