If you already wrote a Letter to the Board for SFC and was selected/promoted, does one have to do it again for MSG board for the same issue?

On the MSG Board, they are not going to read an old letter you wrote to the SFC Board.
If the issue on that letter is still the same, it goes on a new letter to the MSG Board.

You should write a letter whenever you want to clarify something for the board members. It's best not to leave them guessing and trying to figure out a discrepancy in your file

I never had a negative mark on my record, however it was often unclear based on timing as to whether or not something had been submitted on time to be in my records, therefore I always wrote a letter. In your case you last letter does not carry over, nor would it be relevant for a subsequent board. Remember that QMP is almost always reviewed by the same panel. Never leave anything to chance, write a letter. Thank you for your service.