The question Merge 'possible duplicate' comments and votes has status “by design” that manually created “possible duplicate” comments are not merged with automatically generated comments. However it was noticed, that “Folks just typing in "possible duplicate" comments instead of voting or flagging happens on average about 14 times a day”.
It was possibly caused by previous change of lower case ‘p’ to upper case ‘P’ in ‘Possible’ Capitalize "possible duplicate of <x>" properly
This feature request suggests to add extra logic to match on the comment text(case insensitive and ignoring white space differences) and do not create additional comment with the same text, but increment upvotes of existing comment. It will avoid notes like “the comment is a possible duplicate of the preceding one.” Convert variable to type only known at run-time?
Note that according to old questions Possible duplicate comment not showing anymore? (2013)and Auto comment "possible duplicate of ..." missing on voting to close as duplicate (2014) “The comment is only added if the question hasn't already been mentioned in a comment on the post”, but it seems, that implementation has been changed since that.