Copyright notices

[JL VT]

I'm confused on how I should handle copyright notices in files.
What I've done so far is preserve all notices in the files I modify,
and add mine only if my modifications are substantial.

But then I found these classes, all children of CurveOption:
http://www.koffice.org/developer/apidocs/krita-plugins-paintops-libpaintop/kis__pressure__mix__option_8cpp-source.html
http://www.koffice.org/developer/apidocs/krita-plugins-paintops-libpaintop/kis__pressure__rotation__option_8cpp-source.html
http://www.koffice.org/developer/apidocs/krita-plugins-paintops-libpaintop/kis__pressure__size__option_8cpp-source.html

They're SUBSTANTIALLY similar to each other but with different
copyright notices.
In fact, if you look at kis_pressure_size_option and
kis_pressure_mix_option, the only difference, apart from the obvious
difference in class names and methods, is a single included file, and
a "0.5" instead of "1.0".
...how are they so similar yet each one has an only author in the
copyright notice?, It's very likely that someone copypasted a section
out of convenience and replaced words to create the new one (or just
used the old one as a reference).

This leads me to think there are situations where it is acceptable to
put just one's name in the copyright notice, deleting old ones or not
including them if one just copypasted fragments or used the old file
as a reference; is there?.

Also:
When should new copyright notices be added?. If I change a name of a
class, is that worthy of adding a new copyright notice?.

Don't be confused by the nitpicky nature of those questions, I'm not
trying to invent or discover ways to claim code I just copypasted as
entirely mine, I'm just very curious.
Besides, I'm currently studying intellectual property in my
university, so my question is more about the modus operandi of
Krita/KOffice specifically than about a more general, theoretical
reason; because if it were so, I'd give my own input on what the
policy for copyright notices could be  :-)