Student
Action
$4
You may play an Action card from your hand twice. Return this to the supply to gain a copy of that Action
This is a one-shot Disciple. Returning to the Supply is mandatory, so if you use it on a non-Supply card (or a card whose pile is empty), you still get to play it twice, but you lose the Student with no gain, basically turning it into just a one-shot Throne Room
If you Student a Student, then, as with Throning a Throne Room, you get to play two different Action cards twice, however, you only gain a copy of the first Action card played, since you can't return the same Student to the Supply twice. Technically, both Students get returned to the Supply, however, the first Student you played gains a copy of the second Student, so the net result is that you're only returning one Student to the Supply. And the same principle would apply if you chained multiple Students, although you could still gain copies of multiple Actions if you're willing to return more than one Student to the supply, depending on how you order the plays. E.g., with three Students:
Student 1 plays Student 2 twice
Student 2's first play is Hunting Grounds. Play it twice, gain a copy of Hunting Grounds, return Student 2 to the Supply
Return Student 1 to the Supply, gain a copy of Student 2
Student 2's second play plays Student 3 twice
Student 3's first play is Smithy. Play it twice, gain a copy of Smithy, return Student 3 to the Supply
Student 3's second play is, say, another Smithy. Play it twice
So, you returned all three Students to the supply, and gained one Student, one Smithy, and one Hunting Grounds, for a net result of returning two Students for two other Actions
Since it's returned to the Supply and not trashed, you can continue to buy new Students indefinitely. If there are any +buy in the kingdom, it becomes especially easy to replace them - as long as you net at least an extra +buy and +$4 with its replaying, you can just buy a Student to replace the one you returned to the Supply. However, unlike conventional gainers, you can only gain an Action card if you're able to line it up with Student
I'm not sure of the cost - is $4 too low? The fact that you gain a copy of the replayed card (most of the time anyways) is an advantage over Throne Room, but the fact that it's a one-shot is a disadvantage. Is it enough of a disadvantage to justify the $4 cost, or should I make it $5?