Disallow elements that mention the same attribute twice

[lambda-fairy]

Duplicate attributes (e.g. p title="Hello" title="world") are not allowed by HTML5, but Maud still accepts them. It would be nice if Maud emitted an error in this case.

This includes elements which have both a shorthand class/ID attribute and a longhand one (e.g. p.foo class="bar"), as they expand to duplicate attributes as well.

Note that this should be implemented as a hard error in maud_macros, not a lint, since there is no situation where duplicate attributes are useful.