Update statement failed

UPDATE provision
SET    telephone_number='55555555',
       email_address='test@test.com',
       fax_number='124544433'   
where  telephone_number is not null or fax_number is not null
       or email_address is not null
 GO  
Msg 2627, Level 14, State 1, Procedure trg_iu_pr, Line 101
Violation of PRIMARY KEY constraint 'constraint_name'. Cannot insert duplicate key in object 'dbo.provision'. The duplicate key value is (1).
The statement has been terminated.

I checked the table and couldn't find any duplicate values.

Any suggestions???