Have a matching instance variable on your character or whatever is warping, also called warp or playerwarp.
Set the player.warp to match the stairs.warp+1 so it matches to the stairs you want the player to appear at. Do stairs.warp-1 if you want him to go backwards (down the stairs instead of up, etc)
Next compare the values of the variable.warp between stairs and player (for each stairs -> stairs.warp = player.warp)
Then set the player position to the XY of the matching stairs. Then set the player.warp to something like -1 so he can move again to the next stairs. If his varibale remains the same as the stairs he will be unable to move.