A system that stacks amplifiers of effects, while keeping durations separate.

Speed I 60s + Speed II 30s
=> Speed III 30s, followed by Speed I 30s

Run this function as every player that has just joined the game (every time):
function effectstack:join

1. Set "id" to the numeric effect id (supports from speed (1) to hero of the village (32), possible future effects are not supported)
   scoreboard players set id effectstack 1
2. Set duration to the duration of the effect in ticks (20 = 1s)
   scoreboard players set duration effectstack 100
3. Set amplifier to the amplifier of the effect (starts at 0)
   scoreboard players set amplifier effectstack 0
4. Set sid to a "source id" scoreboard players set sid effectstack 0
5. Run v as the player(s) you want to give the effect to:
   function effectstack:effect

If an effect is applied and an effect of the same ID with the same SID already exists, the old effect is overwritten with the new one. If you want to use this system but do not want to use SID, simply add scoreboard players add sid effectstack 1 at the start of the effectstack:effect function and SID will effictively be ignored.

This system will mess up effects if you give them via /effect and then give the same effect via this system

It probably is a good idea to delete any folders from effectstack:apply/tree that correspond to an effect that is not used. (they take quite a bit of space and can use too much ram)

Credit is appreciated, but not required.