If it was indeed from a one-time event you can probably sort by date imported and quickly identify all the dupes. Then you can either remove the old entries and keep going, or remove the new entries and point the DB to the new location of the old entries.
A few things that might make it worth keeping (& updating) the old entries rather than just keeping the new ones and moving on:
- Check a couple (or more) albums to make sure the re-imported tracks have all your tagging - most info is stored in the files themselves but some is stored in the library database. If so you'll have to remove the new DB entries (not the files) and update the DB using "move, rename or copy" and point to the new location. (those instructions are a little vague, let me know if you need more specifics).
- Another thing to consider: any manually-built playlists will have the "old" files in them, if you have extensive manual playlists built that will be another reason to update the DB rather than remove the old entries.