When I heard my son refer to the Kodesh HaKodoshim (Holiest of Holies – the innermost chamber of the Beis Hamikdash) as “Kudshei Kodshim”, Ignoring the slang/accent, I corrected him, explaining that Kodshei Kodoshim are the category of Korbonos (Sacrifices including Olos and Chataos) that are Holier than regular Kodshim (such as Shelamim). Thus they can be plural. But there is only one Kodesh HaKodoshim, and it is Holier than the Kodesh (middle chamber of the Bais Hamikdash). He in turn passed my comment on to his Rebbe, who acknowledged my point.
However, afterwards I noticed that sometimes even the Mishna and Gemara also interchanges the same supposedly incorrect term of Kodshei Kodoshim. So unless there is a Girsa correction that the Bach or other commentators failed to notice, this problem of incorrect Grammer or Word usage has been going on for thousands of years! Or else my observation on which usage is more correct is actually incorrect – but I don’t know why yet.