Multiple [TargetElement] attributes on a single TagHelper should de-dup if multiple match.

[NTaylorMullen]

If I have:

[TargetElement(Attributes = "a")]
[TargetElement(Attributes = "b")]
public class FooTagHelper : TagHelper {}

And I write <foo a="" b="" /> the FooTagHelper class should not be run twice.

[NTaylorMullen]

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