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My take on this is that Minato actually summons other kunai to the seal on the original kunai he throws, much like he summoned his kage bunshin to the original(or was it vice versa?). Thats the only possible explanation as I see it.
If we consider that to be true, then just think of all the possibilities. Him having a seal on someone is like Gae Bolg. Not the causality reversal, but you can just send a knife or whatever to the person you have a seal on. Well, he could do that either way with the S/T barrier, nonetheless.