I think you should stick to MUI
Why is that?
If he wants each player to be able to use such a potion, he can just make it MPI and it will work just fine. MUI will overcomplicate it.
Judging from the variable he used (Attack_Damage[x]), his damage-system is already MPI anyway (and making the potion MUI will not do much in that case).
More information on what he's planning would be useful, though.
If you don't want the potions to stack, you can create a boolean variable that will check if the potion is already active or not.
In case a potion is active, give back the potion to the triggering unit. Else, increase attack damage.
I do know what MUI is, and i can't do that, 2 players are needed to test it.
You don't need 2 players to check for MUI (as 1 player should be able to cast it multiple times).
You do, however, need multiple players to test an MPI-system.
Here's a great resource for that:
KLoader (this resource doesn't seem to be on the hive).