Yes, it's good to ask Our Father for something only once. After that, we should thank him for it when the desire to ask for it again arises. Don't ask again. We should thank him for what we have already asked for.
When I ask, I also follow Jesus' example and I say, "Nevertheless, not my will but thy will be done, Father." I am acknowledging that if the answer is not as I expected, I trust that he has planned something better.
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