Flag certain duplicate elements in XML docs

[drewnoakes]

/// <summary>A summary.</summary>
/// <summary>A summary.</summary>
/// <remarks>A remark.</remarks>
/// <remarks>A remark.</remarks>
/// <returns>Zero.</returns>
/// <returns>Zero.</returns>
public int Foo(int bar) => 0;

Duplicate <param and <typeparam elements are flagged, but not the ones shown above.

There may be a case for allowing duplicates, but I can't see what it is.

[sharwell]

Need to figure out what category to put this in. Most correctness rules have ended up in the Portability category so far, which basically means "not all documentation processors will handle this content in the expected manner".