Is it ok to reheat breast milk more than once? if the baby didn't use it? or should it be does right away?

It's milk!: Whether it comes from a human or another animal, it's still milk. If you leave it out, heat it repeatedly, or if it's old, it spoils. Don't use milk unless it's fresh or it's been stored properly.

You could, but:: Previously frozen milk that has been thawed can be kept in the refrigerator for up to 24 hours. While there is limited evidence to date that milk thawed for a few hours may be refrozen, this results in further breakdown of milk components and loss of antimicrobial activity. The accepted practice is not to refreeze thawed milk. No research about timing of room temp milk "shelf life" (some say 1 hr).