Occasionally you may have been editing a note type and later discover that a number of cards are duplicated (sometimes there may even be more copies).
You can fix this as follows:
- Find one of the duplicated cards in the browser.
- Click the Cards button.
- Select each card type in the top drop-down box in turn, and compare the contents of the template fields and card preview pane. If the card types are identical, delete one of them (Options → Delete).
This should remove all the duplicate cards that use this note type. (If you are still having problems, it's possible that multiple note types have the issue, in which case you can repeat these steps with a card of the other note type.)