Duplicate `@` attributes produce invalid code

[jjonescz]

<input @ref="@a" @ref="@b" />
@code {
    ElementReference a;
    ElementReference b;
}

Produces roughly:

__builder.OpenElement(0, "input");
__builder.AddContent(1, b);
__builder.AddAttribute(2, "@ref");
__builder.AddElementReferenceCapture(3, (__value) => { a = __value; });
__builder.CloseElement();

Notice b is produced as AddContent which won't work at runtime. No warning about duplicate @refs is emitted either.

This happens for other special attributes as well, like @formname.

See also:

razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/IntegrationTests/ComponentCodeGenerationTestBase.cs

Lines 10889 to 10897 in e754229

	public void FormName_Duplicate_CSharpValue()
	{
	// This emits invalid code and no warnings, but that's a pre-existing bug,
	// happens with the following Razor code, too.
	// <input @ref="@a" @ref="@b" />
	// @code {
	// ElementReference a;
	// ElementReference b;
	// }