Add Duplicates validation check option

[atc0005]

Having an exact duplicate of a leaf, intermediate or root is unlikely to be useful, even if clients gracefully handle this scenario.

This should be safe to enable by default as it would be uncommon enough that sysadmins would want to know about this scenario immediately.

See also:

• Add support for explicitly applying or ignoring specific validation check results #64
• Expand certificate chain validation options #120

[atc0005]

Opting for Duplicates as a check name to match the same neutral naming pattern for existing checks and those planning for #123 and #124.