[MU4] Used internal instrumentId is not unique

Steps to reproduce the issue:
1) Start MU and choose "New Score"
2) Select "Choose instruments" and add an Oboe to the score. The list "Your score" shows the "Oboe".
3) Press "Done".

The score contains a single Oboe.

4) Open the "Instruments" form again by pressing "i" or the "Add" button on Instruments palette.

Now the "Your score" list shows a Baroque Oboe instead of the original Oboe.