Sum a Global Property assigned to Map Zones

Hi guys,
I’m trying to figure out how to create a “total income” summary in a Risk-Style game module.
I’ve created zones inside MapBoard and also assigned Global properties inside my multizoned grid:

Zone---Global Property Name: Income Initial Value: 1 "is numeric" min value: 1 max value: 1
Some zones (territory) have an income value of 1 (as the example before) and some other an income value of 2.

I’ve also created a Game Piece Inventory to sum Income values:

I’m using “pieces” on the map to sum zone value and that’s why having 2 pieces create a wrong sum.

Is there a way for me to fix this?

I’d like to sum Zone - Global Property (Income) 1 time only, even if there are multiple pieces of the same colour on that zone.
Is there an easy way to do that?
Please help!

Thank you in advance.