Fascinating, I'm just a lay-person, so this may be completely wrong (and not applicable to specific countries) but it seems that, and correct me again if I'm wrong....
Yes, you can have two Class Actions for the same thing - but both made up of different Plaintiffs.
But then the Defendant can then choose who to deal with which leads to a Class Action Reverse Auction
https://www.overlawyered.com/2005/08/class-actions-the-reverse-auction/
Each Class Action undercuts and reduces the settlement amount/deal in order to gain the advantage which becomes a Race to Judgment
https://www.lexology.com/library/detail.aspx?g=d8fc537a-9bac-42ec-8598-810f8c2e6052
It's a race as the first Class Action to reach judgment/settlement is, by res judicata, applicable to all Plaintiffs.
In addition, the Defendent can go with the Class Action with the plaintiff/s with the weakest case/s to secure the lowest/best deal via an actual judgment that, again, by res judicata, is then applicable to all Plaintiffs.
TL;DR - it seems that having two Class Actions for the same thing is possible, but this can work out to the Defendants advantage.