Ok so if I'm at spawn, because I died in-game, when I run /back, it takes me to my previous location. However if I run /back again it would take me to spawn. This is how /back is meant to work.
My suggestion is to change it so you can do /back list and see locations you have been in, in the last 24 hours. Then you just do /back (number of option)and you get teleported to that option.
The reason why I'm suggesting this is because sometimes you may be at location X on joining, then go to location Y and intend on doing /back but then you die, so you end up going to location Y from spawn when running /back.
Edit: Also I would like to suggest re-adding /getpos too