Report when the same analyzer is passed multiple times

[RikkiGibson]

In dotnet/wpf#6792 (comment) we found a scenario where the build was passing in the exact same analyzer multiple times.

When the same source file is passed multiple times, we give a warning and drop the duplicated file:

roslyn/src/Compilers/CSharp/Portable/CommandLine/CSharpCompiler.cs

Lines 108 to 112 in df86b32

	// warning CS2002: Source file '{0}' specified multiple times
	diagnostics.Add(new DiagnosticInfo(MessageProvider, (int)ErrorCode.WRN_FileAlreadyIncluded,
	Arguments.PrintFullPaths ? normalizedFilePath : _diagnosticFormatter.RelativizeNormalizedPath(normalizedFilePath)));
	trees[i] = null;

Perhaps we should do something similar for analyzers.