Instant Id doesn't change or increment (it stays the same)

i have an assembly containing sub assemblies and parts. one sub assembly is used twice (two piston cylinders).

The problem is that they have the same instant id ,so whenever i change any mates of one subassembly the other changes too , which i don't want to happen

so the question is....

When solidworks enter assemblies or parts with the same instant id and When it varies the ID !?

SolidworksAssemblies