If my opponent uses Mime Jr's "Mimed Games" attack and I also have a Mime Jr. in play, can I choose my "Mimed Games" attack, and if so what happens next?
No, you may not select Mimed Games if your opponent attacks with Mimed Games; you must select a different attack. And if your only Pokemon in play is Mime Jr., then your opponent's Mimed Games won't do anything.
Source: TPCi Rules Team (2024-02-01)

Relevant cards

Note: All applicable cards may not be listed below.
SV45_31
SV45_31