Two piano instruments shown for a single piano part after save/load
1) Create an empty score with e.g. Piccolo and a Piano.
2) Open Parts form (menu -> File -> Parts).
3) Create All parts (using the "All parts" button).
Now tho form look like:
4) Save the score.
5) Close the score.
6) Reopen the score.
7) Open the Parts form (menu -> File -> Parts).
8) Select the Piano part
Now the the piano is shown twice.
Same issue is seen in MuseScore 3.0, 3.3 and 3.4.