Question: Just by a mistake I had deleted a spring project in STS.To use it back I borrowed the same project from my friend in zip format but when I tried to import it says

*Some projects cannot be imported because they already exist in the workspace*

Following is the way I tried to import

file->import->general->existing projects into worspace->select archive file

and after browse when I select the zip project 

*Some projects cannot be imported because they already exist in the workspace*

and the finish button and next button are in disabled state.Please help me

Answer: 
I get this issue from time to time. Usually I just open a new workspace but sounds like you don't want to loose other projects. 

I simply open the.project file in my project and change the name of the project in name tag. 

Good Luck!
the problem is that when you delete a project maybe sts only close it.
Try View Menu --> uncheck closed projects
Now you will see all closed project, simply delete it.