I'm sure that I could find this online, but I don't have time to sift through 150 internet sites of dubious authority :
The other day I was singing through Mass IX (Cum Jubilo). When I got to the Sanctus (Mode V, which starts with a very prominant 3-beat pattern), the melody of 'Resonet in Laudibus' immediately came into my mind. A friend of mine mentioned that it was customary in his parish (and I'm assuming others) to use either Mass IX or Mass X at Christmas. Was 'Resonet' (also Mode V) originally a trope for this Sanctus?
I tried inserting 'Resonet' between the first 'Hosanna' and the Benedictus and it seemed to fit. (This was not during the Liturgy, by the way!)
It does indeed sometimes happen that a melody from the Kyriale and an antiphon sound alike. Take for example the Sanctus (and Agnus Dei) of Ordinarium VIII (De angelis) and the Magnificat antiphon O quam suavis of the First Vespers of Corpus Christi:
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