The German word »einmal« does not always mean »once« in the sense of »one times« (i.e. not twice). It sometimes has other meanings too:
There is the typical beginning of many fairy tales:
Es war einmal ein König, der hatte drei Töchter.
Once upon a time there was a king who had three daughters.
The meaning of the phrase »es war einmal« (»once upon a time«) is not, that it happen only one times (i.e. not twice or more often). I bet, there were other kings with three daughters too. The meaning is, that it happened sometimes in the past. And even in English you use the word »once« although you don't mean »only one times«.
Irgendwann einmal werden wir wieder gewinnen.
Someday we will win again.
With the phrase »irgendwann einmal« you don't say, that you will win only one more times in the future. Hopefully you win more often.
Von wenigen Ausnahmen einmal abgesehen haben alle Bewerber die Bedingungen erfüllt.
With a few exceptions, all applicants met the conditions.
When you are asked to overlook some exceptions, you are not asked to do this just exactly once. It just means, that there are some exceptions.
Ich muss erst einmal darüber nachdenken, bevor ich mich entscheide.
I have to think about it before I decide.
And here the speaker doesn't say, that they has to think about it just exactly 1 times. They just have to think about it.
Er war doch nicht einmal in der Nähe des Geschäftes. Wie kann er es da überfallen haben?
He was nowhere near the store. How could he have robbed it?
And again, it's not reported, that was not one times near the shop. The sentence means, that at the moment of the robbery he was not there.
And so, also in »zuerst einmal« the word »einmal« does not mean »one times«.
Do not learn »zuerst« and »einmal« as two separate words that by accident stand next to each other. Learn this phase how German native speakers use it: as one unit.
English also has phrases that mean something completely different than the single words might let you think. When an English native speaker asks you »what's up?« they are not interested in something that is above them. And when they ask »how is it going?« they are not interested in some mode of walking.