VisualStudio > NuGet: already has a dependency defined for

Error in adding NuGet library.

I opened NuGet manager to add a library for HTTP parsing job while developing an app for test purpose. It's called HtmlAgilityPack, but NuGet failed to install it printing this message below.

NuGet: 'HtmlAgilityPack' already has a dependency defined for 'System.Net' 

To put it simply, having no latest NuGet Manager on Visual Studio 2013 was the culprit.

https://visualstudiogallery.msdn.microsoft.com/4ec1526c-4a8c-4a84-b702-b21a8f5293ca

A link above provides the latest NuGet Manager and it solves the problem right away.

After installing it, you need to start Visual Studio.