You already have the project inside the workspace but eclipse is not recognizing it. In this case, You'll have to re-import the project.
Copy the code from the workspace through your windows explorer (or finder in mac) and paste it outside your workspace.
Then import it again using the code you just pasted outside. Check "Copy projects into workspace" option while importing.
Delete the code you copied outside the workspace.
The project will be re-imported and there will only be one copy inside your workspace.
Also see: