Added possibility to ignore duplicate assemblies

[apazureck]

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Hi, I made another change to catch duplicate assembly / assembly load exception when calling Build(...) on metadatabuilder. The default behavior should be preserved.

I needed this for my module loader to prevent another workaround. Maybe you like it, then pull it, otherwise reject.

Thanks

[apazureck]

Hi, sorry, did not have much time lately. I hope I can put in your changes next week