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: 
Probably whey you 'accidentally deleted' your project, you only deleted it from the Eclipse workspace, but not from the actual workspace folder on your hard-drive (as other people pointed out, Eclipse can arbitrarily map workspace projects to files on disk, so it is possible for a project to be 'deleted' from your Eclipse workspace but still exist on disk.

The good news is the files you deleted are actually still there.

Instead of importing your project from a zip, you may just want to import those files from the workspace folder back into your Eclipse workspace.
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.