Have merge assets by serial number be automatic...

Try checking under Configuration\Solutions Settings\CMDB Solution\Merge Resources\Duplicate Computer Merge.

You can select Serial Number as the Merge Key and enable a Schedule.

I have used this feature and selected serial number as my merge key.   This policy run successfully on schedule, but I am still left with duplicate serial numbers in my database.     I realized after paying closer attention that the default "exclude" collection is "All Computers".   Once I change it to "none", the merge completed as expected.