Most liturgical ingredients in the Prayer Book tradition are set into one particular function with little or no variance. The Daily Office has its own opening and closing sentences, the Communion liturgy has its own acclamations and dismissals, each major liturgy has its own prayer of confession (though the rubrics now allow cross-pollination to some extent), and every canticle has its particular home.
But the Nunc dimittis has two appointments in the Prayer Book, such that it’s even printed in the book twice. It is the second canticle of Evening Prayer, and it is the anthem/canticle in Compline (the night office). This is a particularly odd repetition, since Evening and Compline are both pretty similar in function and usually near one another in time (evening, bedtime).
The reason for this is that in the English Reformation when Archbishop Thomas Cranmer assembled the first Common Prayer Book he reduced the monastic offices from seven in the day and one at night down to two: Morning (Matins) and Evening (Vespers). There are elements of three monastic morning offices wrapped up in Anglican Morning Prayer, and elements of Vespers & Compline wrapped up in Anglican Evening Prayer.
So what happens when you try to bring back in one of those “extra” monastic offices? You get little hints of repetition with the Anglican Daily Office. In the case of the Nunc dimittis its original home was the night office, Compline, which the Prayer Book tradition rolled into Evening Prayer. Thus in modern books that bring Compline back to Anglican liturgy, we’ve got a situation where the Nunc dimittis has two homes.
If this doesn’t bother you, well and good. But if you are someone who regularly prays both Evening Prayer and Compline, and desires to highlight separate identities for both offices, then there is something you can do about that: the 2019 Prayer Book has an appendix to the Daily Office section labeled Supplemental Canticles for Worship. This is an expansion of what earlier Prayer Books (before 1979) did: offering usually two options of a Canticle for each slot (that is, position of response after each reading in the Morning & Evening offices). Now, instead of 8 or 9 total Canticles with specific directions on which ones to use where, we have 15 Canticles, 10 of which provide minimal guidance as to their expected use. If you want to make use of these and reduce the repetition of the Nunc dimittis between Evening Prayer and Compline, consider the following pattern of alternatives:
The Second Canticle, the Nunc dimittis, is to be recited on Saturdays and Sundays, and on other days throughout the year not superseded by the following.
Canticle 4, the Quaerite Dominum, is to be used in its place on the First Sunday of Advent and on weekdays throughout the season.
Canticle 3, the Kyrie Pantokrator, is to be used in its place on Ash Wednesday, the First Sunday of Lent, Palm Sunday, and every weekday throughout the season.
Canticle 9, the Deus misereatur, is to be used in its place on Monday through Friday during Epiphanytide and Trinitytide, and on the occasion that the text of the Nunc dimittis is part of the New Testament Lesson.