The username of the abuse filter system account is controlled by the
localisation message:
https://sr.wikipedia.org/wiki/Медијавики:Abusefilter-blocker
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
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Bartosz Dziewoński