The class "System.Runtime.CompilerServices.ExtensionAttribute" in .NET4.5x has been moved from assembly "System.Core.dll" (.NET4.0 or earlier) to "mscorlib.dll". However, this fact alone probably is not the cause of your problem.
At first, I thought it could be that your solution may have both projects for earlier AutoCAD based on .NET4.0 and projects for AutoCAD 2015 based on .NET 4.5x. However, I created a test solution to one project based on .NET 4.51 targeting Acad2015 and one project based on .NET4.0 targeting Acad2012. Both projects have extension methods. and both projects build OK. If I search Object Browser, I only find one System.Runtime.CompilerServices.ExtensionAttribute (in mscorlib.dll, .NET4.5). In this case, I use C#. That means, to create extension method, I do not have to import namespace "System.Runtime.CompilerServices" as "using System....".
However, I also did the exactly the same 2 projects with VB.NET. In the extension method MODULE, I need to "Import System.Runtime.CompilerServices", and have to explicitly use <Extension()> attribute to decorate the extension methods. Again, the solution still build OK. But search "ExtensionAttribute" in Object Browser yields 2 items as shown in your attached picture. Obviously, you are using VB.NET, and your solution has projects based on .NET4.5x and .NET4.0 or earlier. This could be where the problem comes from, although it is OK in my fairly simple test solution. It also seems C# has its advantage over VB.NET, at lease in terms of doing extension methods
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