Unfortunately, adding a list item to 'My Bookmarks' so it can be reused it on another list is confusing to our users, who instinctively look for a 'Copy' option instead.
To help make this process easier and more understandable, please change the option 'Add to My Bookmarks' in the list item drop down to 'Copy to My Bookmarks'.
This would make more sense and help our users to conceptualise 'My Bookmarks' as being like 'Office Clipboard' --- with it 'remembering' items and enabling them to be 'reused' and 're-added' to lists.
This idea has now been updated.