It would appear that you edited the wrong field--perhaps a field from distributor instead of instrument.
If you have this relationship:
Distributor::__pkDistributorID = Instruments::_fkDistributorID
Then changing the value of _fkDistributorID to a different value should link that instruments record to a different distributor and should not create a new record. But editing __pkDistributorID might. And the distributor name field should be defined in Distributor, not Instruments and thus should not be changed when assigning a record in Instruments to a different distributor.
Hey Phil :).
That solved it indeed. Thank you so much :).