This comes up every year:
If I go to Midnight Mass, and it really starts at Midnight, and I receive Holy Communion, and if I go to Mass again during Christmas Day, can I receive Communion again?
The 1983 Code of Canon Law says:
Can. 917 – Qui sanctissimam Eucharistiam iam recepit, potest eam iterum eadem die suscipere solummodo intra eucharisticam celebrationem cui participat, salvo praescripto Can. 921, § 2. … Someone who has already received the Most Holy Eucharist can receive it again (iterum) on the same day only within the Eucharistic celebration [i.e. Mass] in which the person participates, with due regard for the prescription of can. 921 § 2.
Can. 921 § 2 says that if a person is in danger of death, he may receive Communion even it is not in the context of Mass. That is Viaticum.
That iterum does not mean “again and again”, but merely “again one more time”.
So, if you go to Midnight Mass and receive, and are in the choir at the principal Mass of the Christmas Day and want to receive again, go ahead. But if you stay for a third Mass, nope.