After installing .Net 4 and getting some questions that were already answered here I also realized how the Framework dlls are repeated in several places for the different Framework versions (this is not new, it happens with previous versions, but hadn't paid attention to it until now)
1 - GAC: %systemroot%\assembly
2- Framework installation directory: %systemroot%\Microsoft.NET\Framework\v...
3- and if you have the Windows SDK installed, also in: C:\Program Files\Microsoft SDKs\Windows\
I think the last ones are the so called "Reference Assemblies" and have extra metadata to aid Visual Studio, but
what about location number 2? Why are assemblies repeated there?
No, that's the GAC location for .NET 1.x through 3.5. The GAC for 4.0 is located in c:\windows\microsoft.net\assembly. Why it was moved isn't clear, probably to avoid trouble with projects that referenced assemblies directly from the GAC, a big no-no but it has been done.
Yes, reference assemblies live there. Also in c:\program files\reference assemblies. They are initially verbatim copies of the assemblies stored in the GAC. Until you deploy some kind of hotfix. Keeping them separate ensures that you build programs that target the "proper" framework assemblies, not what you happen to have stored in your GAC.
Yes, no framework assemblies are there, just build tools.
This is more of an educated guess than an actual answer but ...
In order to initially GAC a DLL you need to have a full DLL (aka not-reference assembly) for the GAC to use. The reference assembly won't work as it doesn't have executable code. Hence you need a real DLL in which to source the GAC so you get location #2.
来源：https://stackoverflow.com/questions/2851406/why-are-framework-dlls-repeated-in-several-places