NuGet: 'X' already has a dependency defined for 'Y'

This was resolved by installing the latest NuGet Package Manager:

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

Don't forget to restart Visual Studio.

I fixed a similar issue in my solution by:

1. Opening up a command prompt
2. Navigating to the .nuget folder in my solution
3. Running nuget update -self

This upgraded the copy of NuGet.exe that was in my solution from 2.8.0 to 3.4.4, which fixed the 'X' already has a dependency defined for 'Y' error that was stopping it from downloading SSH.NET automatically before building.

(If your solution doesn't have a copy of NuGet.exe in it - and it might not - then you should try the solution in TN's answer instead)