Multiple instances of flying, vigilance, and/or lifelink on the same creature are redundant.

If you control two On Serra's Wings attached to two creatures you control with the same name, the "legend rule" applies to the enchanted creatures and to On Serra's Wings at once. You can choose to keep the On Serra's Wings that enchants the creature you wish to keep.

If you control two permanents with the same name but only one is legendary, the "legend rule" doesn't apply.