<File> is already in this project; you do not need to share it.
This warning has the following cause and solution:
You can’t have two items in a project with the same name. You are attempting to share a file into a project that already contains a file or subproject with the same name.
If you would like the file to be in the project, you will either have to:
Rename the existing file/project before sharing the file into the project, or
Delete or destroy the existing item from the project