It could be called plot inconsistency but I rather not think of it like that.
- '|| J_prev at t = -5
|---- Where J current will end up after jump at t = -5
o--== J current at t = 0 __
Now J decided to go back a few minutes to when Boris just threw him, this would place him exactly at the stop where
J_prev would be at. Without trying to get too deep into space-time continuum, the time device moves you by time not space. So you are in the very exact spot, two steps (plus or minus) before you made the jump.
Then, one could see the situation as two bodies being at the same exact spot at the same exact time. So there can only be one mass that can occur at that spot.
Building on that, J uses the device to jump the timeline so in order to maintain consistency without breaking anything after he finished saving the world with K he would jump back to the time he left (on the building). If he were to jump again while not returning to complete the first jump in 2012, which timeline would he be in? A paradox would occur in my opinion. The most logical thing for the writers to do would be to have J enter his old self on the second jump.