The following message a project with that name is already opened in the solution when you try to add -> existing project ... to your solution.
dependency : AnkhSvn

This is an problem with AnkhSvn for which the following work around works well:

be careful and please backup/make a copy of your solution file before proceeding:

open your solution file with notepad or similar and search for the reference to the existing project:
(mine looked as follows)

Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "GraspTest", "GraspTest\GraspTest.csproj", "{28F01283-3324-4E3C-A1FE-C256F29130A6}"
EndProject

Just delete this entry and try again.