If there is nothing that distinguishes the original from the duplicate, there is no way to gather them together automatically. Is there something that distinguishes all the duplicates (in iTunes), such as a Date Added that is after a certain date (for the duplicates)? Or play count (Plays) that is zero. If you can find something the clearly and reliably distinguishes the duplicates from the originals, you can create a smart playlist using that "thing" and gather them together on one list automatically. You can then Option-Delete them from the playlist to remove them from the playlist AND your library, in one shot.
Even if that's not possible, you can make it easier. First, hold down the Option key while selecting to Show Duplicate Items from the menu bar. That command changes to Show Exact Duplicate Items. That should give you a better list.
Next, you can select multiple items at the same time, on a list in iTunes. To select consecutive items on a list, click first item, hold down Shift key, and click last item. The two clicked items and all items in between are selected at the same time.
More useful for this situation is selecting non-consecutive items. To select separated items at the same time, hold down the Command and Option keys, and click items to select them. All clicked items are selected at the same time. For this situation, you're probably clicking every other item on the duplicates list.
Once a "batch" of songs to delete are selected at the same time, right-click the selection and Delete. Not automated, but you don't need to delete songs one at a time.
