At Mass this morning I received Communion, then unexpectedly invited to serve Mass this evening, which I did. I did not think to say to the priest before Mass that I should not receive, having done so already, [That's okay.] and to avoid disrupting the liturgy ended up doing so for a second time in the day. [You do not disrupt Mass by receiving Communion!] I will mention this when next in the confessional, [No need.] but is this a sin or is receiving twice acceptable when serving? Both Masses were Extraordinary Form.
Please please please remember: Receiving Holy Communion during Mass, server or in the pews, does NOT disrupt Mass. I know that some traditionalists have this view. As a matter of fact, I have been in choir (sitting on the side in the sanctuary) at Masses of a particular group and, when I have indicated I would like to receive, was entirely ignored… as a priest. But I digress.
We have got to eradicate the idea that somehow your reception of Communion interferes with anything.
Next, it doesn’t make the slightest difference whether this was the Extraordinary Form (TLM) or the Ordinary Form (Novus Ordo). It doesn’t make a difference if you are serving or in the pews.
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 in the morning you were at some Novus Ordo Communion service wherein you received Communion, or a Mass in either Form. Later in the day you stumble into a church where Mass was about to be celebrated. At that Mass you could receive Communion again (iterum). However, 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 because a Communion Service isn’t Mass.
Canon 917 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.
So, do NOT worry about this. You did NOT sin in receiving Communion, during Mass, the second time. You did NOT disrupt Mass by receiving Communion. And if the priest gave you to think you did, he should be admonished by proper authority in the strongest terms to knock it off.