Can. 917 One who has received the blessed Eucharist may receive it again on the same day only within a *eucharistic celebration in which that person participates, without prejudice to the provision of can. 921 §2.
Can. 921 §2 Even if they have already received holy communion that same day, it is nevertheless strongly suggested that in danger of death they should communicate again.
*The Canon Law Letter & Spirit * commentary on Canon 917 says the following:
“The 1917 Code at cc 857, 858 §1 allowed the reception of holy communion a second time on the same day only in danger of death or in order to avoid irreverence to the blessed Eucharist. Between the end of Vat. II and the Code this regulation was relaxed. After the promulgation of the Code there was some debate as to whether the word ‘again’ meant twice only or as often as a person attended Mass. A reply from the Code Commission in 1984 made it clear that it means twice only. Consequently the present law is that a person may receive holy communion a second time on the same day, but only on the occasion of participating in a Mass—unless of course there is danger of death.”
Concerning the Eastern Catholic Church law on this issue, Colin B. Donovan, STL says, “In the Eastern Catholic Churches the practice of frequent daily Divine Liturgy (Mass) is not found, and so the Code of Canons of the Eastern Churches is silent on this matter."
Excellent site on canon law.
*Note: “Eucharistic celebration” means a Mass not a Communion Service