This addon enhances the Add Card screen with autocomplete functionality. Just start typing and if an existing card has a value that matches what you've typed, it will be displayed inline in the editor for you. From there you can click the value to quickly set the field.

However the real reason I wrote this plugin is that I'm playing through a game for the second time, and I want to be notified immediately of cards that I'd already added the first play through. In the following screenshot you can see I didn't have to type the full sentence to see that I already had it in Anki. In such cases I'd simply delete what I'd started to add, then resume playing. This is much quicker than typing in the full sentence (in exactly the same way, especially the punctuation) and looking for the "Show Duplicates" button.

The autocomplete search will only search the exact note type and field that you're typing into (so you won't get results from other note types or even other fields in the same note). The search also only does prefix matching, like most other autocompletes.

Psst, you may also be interested in the similar Frozen Fields plugin. :)