This is a good question. My gut-reaction answer would be that if you've shuffled for any reason, that would count - no need to do it again.
I went through the rulebook and found two related sections:
1) This part makes it sound like you shuffle at the end of the round NO MATTER WHAT following a Null/2x draw. There are no exceptions listed.
End of Round
If a standard “2x” or “Null” attack modifier card was drawn from a specific modifier deck
during the round, shuffle all the discards of that deck back into its draw deck.
2) However, this part makes it sound like the end-of-the-deck reshuffle could account for and override the Null/2x shuffle.
When it says, "This shuffling also happens...", it makes it sound like one instance of shuffling only, regardless of the cause/trigger.
Alternatively, the inclusion of the word "also" could mean you have to do both shuffles. Hard to tell!
Attack Modifier Cards
At the end of the round in which a “Null” or “2x” card is drawn from a deck, players will
shuffle all the played modifier cards back into that particular draw deck.
This shuffling also happens if a modifier card must be drawn and there are none left in
the draw pile.
Really though, this is another complexity that is not addressed in the rulebook, and only the creator would have the definite answer.