The username of the abuse filter system account is controlled by the localisation message: https://sr.wikipedia.org/wiki/%D0%9C%D0%B5%D0%B4%D0%B8%D1%98%D0%B0%D0%B2%D0%...
The translation has been changed in 2020, which is why there are two accounts: https://translatewiki.net/w/i.php?title=MediaWiki:Abusefilter-blocker/sr-ec&...
They can't be merged, but you can remove the user rights from the old one.
There is a bug about making these accounts use the same username and a global account on all wikis: https://phabricator.wikimedia.org/T160666