In a medium bowl, combine the diced mango, pineapple, soy sauce, honey, olive oil, minced garlic, ground ginger, black pepper, red pepper flakes (if using), and lime juice. Mix well to create a marinade.
Place the chicken breasts in a resealable plastic bag or shallow dish and pour the marinade over the chicken. Seal the bag or cover the dish and refrigerate for at least 1 hour, or up to 4 hours for more flavor.
Preheat the grill to medium-high heat. Remove the chicken from the marinade and discard the marinade.
Grill the chicken for 6-7 minutes on each side, or until the internal temperature reaches 165°F and the chicken is cooked through.
Remove the chicken from the grill and let it rest for 5 minutes before slicing.
Serve the grilled chicken topped with the remaining diced mango and pineapple, and garnish with fresh cilantro.