What makes an Asset 'Unique'

I'm having a small problem with 2 assets I'm trying to scan, they both get scanned perfectly fine and all info is updated but some way they cannot be both in the list of assets.

If I scan Computer A all the info gets updates, I assign it and it appears successfully in the assets. Then when I scan Computer B all the info once again is updated perfectly, including IP - hw & softwareinfo's, but it replaces Computer A in the assets and keeps the assignment of Computer A

This got me wondering on what exactly makes an asset unique?
- model
- service tag
- product name?

Computer A and Computer B are completely the same workstations but only different ram & software installed. A product name or servicetag are not present however there are more likewise workstations in the domain who seem to be added just fine without replacing any other machine.

Does anyone have an idea where its going wrong?