It'll happen. The schedule prefers not to have the same type of advent be held consecutively (collaboration advents are an extra here; they're active for longer and concurrently with the 'regular' events).

Let's take a quick peek at the next.... 9 events on DMM's schedule to see an example:
Wrath - Union
Sphinx - Advent
Envy - Union
Jack o' Frost - Advent
St. Nicholas - Raid
Garuda/Typhon reprint - Advent, but we already got this
Reiki - Raid
Gluttony - Union
Qiongqi - Advent

After Qiongqi? There's like a 4 and a half month period in DMM's schedule that's composed of raids, unions, and reprints we already got ahead of time.
So, what happens when we hit points in the schedule where a reprint was originally ran?