I found that there are currently two domains for the wiki that are a mirror "copy": https://wiki.lazarus.freepascal.org
https://wiki.freepascal.org
 
I didn't know this, and for a long time I was wondering why some pages I bookmarked didn't have a "star" icon in the browser. Later I found out that there are many copies of the same pages in the bookmarks. I'm sure I'm not the only user who will make such a discovery.  
Why did it happen there? Was there ever an idea to separate the IDE and FPC wikis? Or were they merged at some point?  
In any case, now I think it's better to add a redirect, rather than leave a "copy" of the site. I'm sure it will save users from confusion, both with bookmarks and with domains (since at first I thought these were separate wikis for IDE and FPC).  
It will also save users from having to log in to the same wiki twice on each domain.

Why did it happen there? Was there ever an idea to separate the IDE and FPC wikis? Or were they merged at some point?

Yes, originally the wiki was started for Lazarus. Later on, FPC got added. IIRC, originally you got a different main page depending on which domain you used to access it.

Removing one of the domains would also break a bunch of people's bookmarks though.

Jonas

I am considering the option of redirecting, not deleting the domain. That is, so that all pages from https://wiki.lazarus.freepascal.org are automatically redirected to https://wiki.freepascal.org.  
All bookmarks will continue to work. It's just that bookmarks from the "lazarus" domain will no longer be "marked with an asterisk", but in the long term (when adding new bookmarks and for all new users) more and more of them will be on the "freepascal" domain, so this problem will no longer be relevant.  
By the way, when you open just the "site" (without specifying the page), such a redirection already exists... You can check by clicking the links above. But if you specify any page (even the main one), then each of them remains on its own domain, for example: https://wiki.freepascal.org/Main_Page
https://wiki.lazarus.freepascal.org/Main_Page