Mine is the same in that if I select a title clip that is already on the time line, the Edit > Duplicate is grayed out.
When I create a title with the Title tool, it does create a title in the Project Assets box. As far as I know, you can't create a title (or any other clip) without it being in the Project Assets box. However, it can hide in a folder or beyond the visible window. Try grabbing the right side or bottom of the Project Assets box and enlarging it. The title icons look almost exactly like the clip icons.
Regarding title duplication...... You can drag a title from Project Assets to the time line in multiple places. But, if you try to change one of them, they all change. The only good way to duplicate a title is in the Project Assets box. Then you can make changes to the new copy as needed. To do that, select the existing title, right click and select Duplicate.
Good luck! If you used the Title tool to create your title clips, I'm sure they are in the Project Assets box.
Bill