Machal

I have a global event that every 5 seconds adds 1 currency e.g. activity...

Using one simple node, that checks, if players inventory has 0 activity, if yes, then it adds 1. If it has 1, then it does nothing.

Turns out it keeps adding the currency on and on...The way this node works is weird as it snaps the number first time and uses it every time the global event repeats.

But that's not how it is described to work...

gamingislove

Has In Inventory checks for having at least the defined quantity, not it being exact.

You'll have to change the setup to check for 1 quantity and add 1 in the Failed slot.