This isn't something that happens by default.  There must be something more to it...  🤔 

Are you using the "Rich Text Box" setting, or "Basic Text Box"?  This only happens when you choose the Basic Text Box mode, then edit using the Text render mode.  If you use the HTML render mode with Basic Text Box, there are no additional line breaks added.  

Can you post an example of what you enter and what is saved?

Thanks very much for responding on this, Will and Timo! I'm sure it's some weird misconfiguration in my install.

Will - when I open the editor, it shows I'm using the Rich Text editor. I never switch that back and forth - but it does seem to be a good explanation that it's converting new lines in the source into CR/LF characters.

Timo - I'll attach an image showing the before and after, as shown in my external code editor so that I can show the special characters added. The top 12 lines are the original source I copied into the editor's Source pane. I then saved, then went back in to edit again. The bottom part from line 13 down shows what has been added after saving and reopening. The added characters do not affect the rendered HTML display, they are just CR/LF characters. (just to be clear, I pasted both the before and after examples one after the other in this external code editor for convenience).

I'll check through my ckEditor configuration again and see if I can find anything regarding Basic Text box vs Rich Text settings.

Ah, that's indeed interesting.  I haven't seen that one yet myself... Have you, Timo?  

I saw something similar with an old site, back at version 5 or 6 I think that was then upgraded. Is this an upgrade to a relatively old site?
Unfortunately I can't remember exactly how I resolved it, though I vaguely recall it being adversely affected by custom IIS rewrite rules that were designed to "fix" presentation of some content - or it might have been due to a web browser plugin to "help" spelling??