"Mamayâ" is "later today" while "mayá-mayá" is "a few minutes from now" or "a little later".
"Si Tom ay pumasok sa trabaho. Ang uwi niya ay mamayâ pa". = Tom went to work. He's coming home later yet.
"Si Tom ay pumunta lang sa tindahan sa kanto. Nandito na 'yun mayá-mayá = Tom just went to the corner store. He'll be here in a short while.
In this example, "mayá-mayá was used because the idea being conveyed is that right after frying the fish, it will eaten for lunch. Without mentioning it, the time that Mary is frying the fish will be understood to be just before lunch time.
Had this instead used "mamayâ" and Mary was frying the fish close to lunchtime, it would most likely be understood that the fish is to be eaten for supper, i.e., later today yet.