Something seems to have gone a bit wonky. Did this only happen for me?
Stack Exchange launched new instructions for review queues recently. See New onboarding for review queues on the main meta site.
Everybody is getting the pop-ups once per Stack Exchange site, regardless of whether or not they have just earned access to the queue, or have already completed thousands of reviews. Catija ♦ says:
Many people already have the ability to access these queues and don't use them or use them poorly - so we want everyone (including people like me) to be aware of the new help guidance. Now, whether they'll look through that guidance if they're in the latter group is an open question but I think it's still worth making it available to them.