Fix TS generator crash from duplicated reference assemblies

[jasongin]

When provided duplicate reference assemblies, the type-definitions generator tool crashed with the following stack trace:

Unhandled exception. System.ArgumentException: An item with the same key has already been added. Key: <assemblyname>
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Microsoft.JavaScript.NodeApi.Generator.TypeDefinitionsGenerator.GenerateTypeDefinitions(String assemblyPath, IEnumerable`1 referenceAssemblyPaths, IEnumerable`1 systemReferenceAssemblyDirectories, String typeDefinitionsPath, IDictionary`2 modulePaths, String targetFramework, Boolean isSystemAssembly, Boolean suppressWarnings, IEnumerable`1 excludePatterns) at TypeDefinitionsGenerator.cs line 273
   at Microsoft.JavaScript.NodeApi.Generator.Program.Main(String[] args)

De-duplicating the list of provided reference assembly paths fixes it.