If the duplicate projects are identical you can delete one without affecting the other. As noted, you would need to open and play them to be sure. By just deleting a project you would not be deleting the original media that the project is referencing, unless you went to the Event that contains the media and deleted the Event. So if both projects are referencing the same original media in the iMovie library, then deleting one project won't affect the other as long as you don't delete the original media.
I can't tell whether the procedures you mentioned would create an original and a duplicate in one library. Possibly so. You could try with a test project and see.
If you send a copy of a project to a different library, it would have the same identical name in both libraries without a number added to the title unless there were already a project in the destination library of the same name. When you create a duplicate in the same library then that duplicate would have a number added to the title to distinguish it from the original.
Always a good idea to do a File/Consolidate Project Media before transferring or copying a project to another library. Likewise if you are borrowing clips from one library to use in another library, do a File/Consolidate Library media on the destination library after making the copy or transfer.
-- Rich
