Inventory Quantity functionality

Summary

when install message scenario is triggered twice for the same activity and same inventory item , quantity is being added

Content

Hello OFSC experts, 

I am facing an issue with field "quantity" . Any advice is welcome.

For example: 

1)Available items for a specific non-serialized inventory are 5. 

2) Technician inserts 2. 

3) Install message scenario is triggered with quantity 2

4)At the same activity and same inventory, Technician installs another 1.

5)Install message scenario is triggered with quantity 3 instead of 1.

Actually, when install message scenario is triggered twice for the same activity and same inventory item , quantity is being added ( 2+1)

Can you provide any solution for quantity value in order to work independently ?