Later Edit:
I found it! The setting was there in preferences/searching: Highlight search results....
It was checked and I am 99,99% sure I haven't done this.
Problem was solved by unchecking it.
I'm not sure if I should delete whole post, or leave it in case others have this problem.
I am also not sure if indeed Duplicate Finder modified this setting, in case it absolutely did not the this is wholly off-topping and I'm sorry for that.
L.E.2:
I see my problem was already mentioned buy I did not know the corect wording of that so my searches did not get me to that page:
https://www.mobileread.com/forums/sho...d.php?t=237884
So it seems that would confirm that Duplicate Finder does modify that setting.
In my humble opinion, a warning about that on it's download page would be nice (if it's already there then it must have escaped me somehow).