A reader in a comment under another entry asked a question about reception of Holy Communion.
Can [you] communicate twice if one Mass is 3rd Sunday of Ordinary Time and the second one is Conversion of St. Paul.
For Latins 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 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, say you were in the morning at some Communion service wherein you received Communion. Later in the day you were while driving happy to stumble on a church where Mass was being celebrated. You could receive Communion again (iterum) at that Mass. However, if the opposite were the case, if you were at Mass in the morning and then stumbled into a Communion service at a priest-less parish in the afternoon, you could NOT receive again: it isn’t Mass.
The canon tries to walk the line between promoting frequent reception of the Eucharist and a superstitious or excessive frequency. The key here is that the second time must be during a Mass, unless it is as Viaticum and you may not enter the Mass at some late point merely to receive.