OK, this is where my less-than-fluency bites me in the ass. So, موفق is the noun/adjectival form (the difference is in short vowels that aren’t usually included; you figure out which form it is from context). It means “serendipitous,” or “chance,” or, in my brain, “lucky.” حظا is “luck.” No ambiguity.

Add the alif to the end of موفقا, and you’re either indicating a case change, or making it adverbial (or you could be making it a dual–referring to two people, but that would be weird). It’s hard to tell, because the difference between the case change and the adverbial are diacritics that, like short vowels, are usually not included and you infer them from context. If it’s an adverb, that would mean that the alif at the end of حظا isn’t there to mark case, but to indicate that it’s the adjective form. So it’s not “luck,” but in fact, “lucky.” And the موفقا is an adverb modifying the adjective “lucky.” So you can’t be “lucky lucky.” You’re “serendipitously lucky.”

Which would be the best type of luck to have, right? That’s where your “best of luck” came from.

This is actually the type of thing that drives me away from Arabic. I remember working hard on translating some stuff, having it not make sense, and then working for hours to hammer it into some coherent form. Except that the third word of the piece I was translating was a word that is colloquially used for “camel,” and my translation was not only 90% wrong, but 10 times as difficult as it would’ve been.