(Uh, that error doesn’t seem like something I caused…)

It looks like this plugin completely ignores archives. You can easily have a blog which has indexes already there.

For example, my blog has https://chriswarrick.com/blog/2014/ and also https://chriswarrick.com/blog/2014/11/09/inbox-review/ via archives and posts/ structured mirrored in output.

Neat. How did you get https://chriswarrick.com/blog/2014/11/09/inbox-review/ to show up there? Is the inbox-review post in a "2014/11/09" directory?

(Also, that seems to solve a slightly different problem than the one I was trying to solve, since I want the sub-indexes to show up, and https://chriswarrick.com/blog/2014/11/ doesn't show all the posts in 2014/11… :)

1. Yes, the path is posts/2014/11/09/inbox-review.rst (/blog/ is handled by POSTS)
2. 2014/11 does not work because I didn’t enable CREATE_MONTHLY_ARCHIVES, which is overkill for small blogs. I could also enable ARCHIVES_ARE_INDEXES and CREATE_DAILY_ARCHIVES for a better effect.
3. Still, your plugin should check that it doesn’t conflict with archives. Otherwise, a nice error will prevent the site from building due to two tasks having the same target.

Ah! Yes! I should definitely make it check for that.
Is there a clean way to say "Does this target already exist?" (or "Is another plugin going to handle this target?") so that I can skip generating it? (That would let me avoid the check at https://github.com/bwinton/plugins/blob/master/v7/subindexes/subindexes.py#L58 too!)

No, you’d have to check possibilities manually, according to current archive settings.