Either you have this project in your Eclipse workspace already (a), or it was added to another working set (b).
a) To fix this issue you need to delete project from Eclipse Workspace. Open Workspace folder, find project there and delete it. Afterwards you can import project again. To find out Workspace folder open File - Switch Workspace. You will see workspace folder there.
b) Press small triangle button in the Project Explorer (see picture) and choose Edit Active Working Set option. Find your project and add it to working set content.
Hope this helps.