Historically bots and humans fix double redirects in MediaWiki, not the software. For the bot part we have https://phabricator.wikimedia.org/T77025

The issue with double redirects is slightly different within wikibase.

When using wbgetentities (or probably any api module) with the ID of a redirect everything is fine, you edit the target of the redirect.

If you do this for the ID at the beginning of the chain of a double redirect then instead the entity shows as missing.

Either the software should automatically avoid double redirects when creating them
OR
The software should be able to navigate through multiple levels of redirect

Well AFAIK the software now doesn't allow for redirecting to a redirect, a RedirectCreationException is thrown with the code target-is-redirect

Personally I think the maxResolutionDepth of EntityRedirectResolvingDecorator should be set to something greater than 1, perhaps 5?
This will avoid the issue described above and shouldn't impact anything greatly.
Bots would continue to run to fix redirects

The alternative would be on creating a redirect, find everything that is already redirecting to the itemid that you are about to redirect to somewhere else and also change those....

Thoughts @daniel

I don't think auto fix double redirects is a good idea.

For example, there're 1000 items redirected to item A and 1000 items redirected to item B. If we merge item A and item B, we must at least fix 1000 pages.

Also redirects are automatically fixed by bots.

It's however useful to increase $wgMaxRedirects in case double redirects break other services (wbgetentities, usage tracking, RDF dump, ...)

In T71167#1216284, @Addshore wrote:

Personally I think the maxResolutionDepth of EntityRedirectResolvingDecorator should be set to something greater than 1, perhaps 5?
This will avoid the issue described above and shouldn't impact anything greatly.

It should be $wgMaxRedirects, which is 1 per default and on all WMF sites, afaik. Would be interesting to know if there would be any issues with increasing it. However, I see no reason to treat wikibase items specially here. Item redirects should behave e3xactly as all redirects do (and this is currently the case, as far as I can tell).

The alternative would be on creating a redirect, find everything that is already redirecting to the itemid that you are about to redirect to somewhere else and also change those....

That's what $wgFixDoubleRedirects controls. It's disabled on WMF sites I think. The documentation says: "Fix double redirects after a page move. Tends to conflict with page move vandalism, use only on a private wiki."

It would be nice if RedirectCreationInteractor would optionally fix double redirects, if $wgFixDoubleRedirects is set. That way, Special:MergeItems would behave in a way that is consistent with what Special:MovePage does.

It would be nice if RedirectCreationInteractor would optionally fix double redirects, if $wgFixDoubleRedirects is set. That way, Special:MergeItems would behave in a way that is consistent with what Special:MovePage does.

That sounds like a plan