SubForm tries to insert primary key twice. duplicate error happens

[Tom Frink via AccessMonster.com]

I have a query that joins multiple tables. This is used in the subforms record source to retrieve records. Im joining the primary key from the master to the child tables to link the forms. For some reason My subform wants to insert a new record instead of updated the record in the table. They say it works for 1 to many but I dont want new records in the table but only to update the primary if its already been entered by another subform.

Maybe I should do some sort of lookup to see if the primary key exists then if so, update the record otherwise do an insert.

How would I do this without effecting the text box and the value it shows.

Thanks
Tom