Check and report duplicate references

[antoniogamiz]

This issue comes from Raku/doc#1912 and it's also present here.

[antoniogamiz]

Do we consider references with the same display text, duplicated? @JJ

[antoniogamiz]

After Raku/problem-solving#216, I think is a good idea to resolve this problem as follows:

• Throw a warning for those references with the same name but different URI. These references are valid and point to different places, so the only problem is that they will appear "duplicated" in the search bar.
• If two or more references are found with the same URI, then documentable will fail, listing all duplicated references found and where they are located.