When receiving consumables from a purchase order line in the procurement module it is doubling the quantity in the stock room.

Changes made to out of box business rule 'Reset Assigned To'.

The behavior seen is due to changes made to the out of box business rule 'Reset Assigned To'. Please see the link below:
<Instance name>.com/nav_to.do?uri=sys_script.do?sys_id=81954b03c30331004e44dccdf3d3ae72%26sysparm_view=sys_script_action

In an out of box instance this is set as a before business rule. This has been modified to an after business rule, with a call to current.update(). Due to this update call, the business rule which updates the quantity field ('Merge records') is called more than once. In turn this is updating the quantity field twice.