Yep, Sync is probably causing that duplication. Are you using Sync with Firefox for Android, in addition to Firefox on laptop / desktop devices?

nope, just desktop/laptop...

You can check for problems with the places.sqlite database file in the Firefox profile folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

• Places Maintenance: https://addons.mozilla.org/firefox/addon/places-maintenance/

If you use Sync then best is to disconnect Sync (temporarily) while performing maintenance/repair efforts to the places.sqlite database.

Use with Extreme Caution;

Bookmark Duplicate Cleaner 0.2 by Anders Lindmark Bookmark Duplicate Cleaner {web link} Scans for bookmark duplicates and allows you to delete them

Bookmarks Checker - check for bad links by FVD Media Inc. Bookmarks Checker - check for bad links {web link} Check favorites for broken or bad links. It checks for bad links and displays a report if it finds something. Check for 404 links, timeout links.

thanks all for the replies

that bookmark duplicate cleaner fixed it... select the bookmark,. shift end (apparently a couple of times.. too many to load) and delete

I trust you cleared the duplication from all Synced devices. Otherwise Sync will add try to add them back again, when turned back on, and probably generate even more corruption.

You may want to make routine occasional manual backups of your known to be good sets of Bookmarks.

• Restore bookmarks from backup or move them to another computer

Sync sometimes corrupts or duplicates bookmarks. That is a known issue and unlikely to be fixed. The problem is most likely when there are a lot of changes or a bad internet connection.