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Yes, it is legal. This is because neither of those actions is a criminal conviction. The law concerning "double jeopardy" states that you cannot be convicted in the same court for the same offense twice.
A GOMAR is not legally considered a punishment and it certainly is not a conviction. It is considered an administrative process and that's it.
An Article 15 is non-judicial punishment, so while it is considered a punishment it is not a court punishment and double jeopardy doesn't attach to it either.
Where I have seen this occur, I have seen defense counsel successfully argue that because one has already received the GOMAR, which is a reprimand, it is cumulative to add another "reprimand" in the Article 15 itself (which is one of the potential punishments in the Article 15). That, however, is the only restriction I've seen successfully used.