Improve hexdoc handling of duplicate patterns when allow_duplicates is enabled

[object-Object]

In #911, we added an allow_duplicates option to [extra.hexcasting] in hexdoc.toml. The goal of this feature was to allow users the option to prevent their hexdoc build from failing if they add a pattern with a duplicate signature (mainly for HexBug). However, the implementation in #911 is flawed: duplicate patterns are not added to self.patterns, resulting in a later validation failure for pages that use the duplicate patterns.

This PR changes the logic as follows:

• Always raise an exception if a duplicate pattern ID is detected, since there's no valid reason for that to happen and we need pattern IDs to be unique in order to look up patterns for hexcasting:pattern pages
• If a duplicate pattern signature is detected and allow_duplicates is enabled, still add the pattern to self.patterns so we can look it up when loading pattern pages
• Improve the formatting of the error messages for duplicate patterns

These changes have been tested in HexBug and seem to work fine.

[Robotgiggle]

LGTM