Add a warning if a heading title is used in multiple places

[samvanstroud]

This should cause mkdocs build -s to fail.

[pawamoy]

Hi @samvanstroud, we just merged #39, which allows to define unique aliases pointing to specific headings. I believe this solves the issue described here (headings appearing in multiple places): since we can now define unique aliases, duplicated headings pose no issue and shouldn't emit warnings.

[samvanstroud]

Hi @pawamoy, thanks for the feature! This is great.

What happens if aliases are not defined but multiple headings exist? i.e. in the case that the user either 1) doesn't know that a title is being multiply defined or 2) doesn't know about this feature. In that case I think a warning should still be emitted.

[pawamoy]

OK let me reopen. It might be possible to detect when a heading is used multiple times while not having a unique alias pointing at each occurrence.

[beilm]

I had the same problem and made a PR. Maybe that will help here too?
#50

[pawamoy]

Thanks again @beilm!