Extra copies of "activate only once each turn" abilities have their restrictions tracked independently. That being said, you only apply each of the ability stealing effects one time so you don't end up with infinite.

For example if you had two of the same card that says, “if a blue triggered activates copy that ability,” on a blue card. You’d get the initial trigger, the two copies of that trigger, and then two copies of the copy of the trigger, and then then two copies of the cards copying each other. So in sequence you’d have: Initial Trigger Blue A Copies IT Blue B copies BA Blue A copies BB Blue B copies IT Blue A copies BB Blue B copies BA So you’d have six copies and the initial trigger for a total of seven.

You could also change it to stop all infinite combos and with something like “you can’t trigger from something that has already caused that trigger”. But with better wording as that would make that time I accidentally went infinite with my fungus deck. I had life and limb and sporemound in play and then played a land without thinking about it and poof a never ending cycle of saprolings.

You actually don't need a rules change. Each continuous effect is only applied once and you apply them in timestamp order. Marvin steals 1 copy per ability, then Sakashima steals 2 since it sees Marvin. Then you are done.

When a card steals abilities like this it treats instances of the name of the card it stole from as if they said it's own name (except for ones like "cards named ____"). Marvin only steals activated abilities, not static ones. 201.5a: If an ability's effect grants another ability to an object, and that second ability refers to that first ability's source by name, the name refers only to the specific object which is that first ability's source. The second ability does not refer to any other object with the same name as the first ability's source. However, if the second ability also moved the first ability's source to a different public zone, the name refers to the object the source became in its new zone. This is also true if the second ability is copied onto a new object.

@@seandun7083 So it Marvin gets played, imposter copies it, then Marvin loses the static ability all while a llanowar elves is out. It would be only the imposter that can tap for a green? It's confusing bc the static ability says Marvin gets the activated abilities, not "this creature"

@@peterbruno657 yeah. If you Unable to Scream Marvin then that's what happens. Sakashima changes the ability to "Sakashima has all activated abilities of..." It's a similar reason to why activating a Shivan Dragon doesn't pump every creature named Shivan Dragon, but only the one whose ability you activated.