Many plain URLs redirect to add the www subdomain (e.g. google.com becomes www.google.com). If such a bookmark is imported and opened, the Raindrop.io interface treats it as though not bookmarked, and the Raindrop button will create a duplicate bookmark that has the www.
Reproduced using the Chrome extension, Save Button for Web.
Steps to Reproduce:
1. Save https://www.google.com/ to raindrop
2. Export bookmarks as html. The exported file correctly has www
3. Import the html file. The new bookmark does not have www
4. Open the bookmark. The button indicates that it isn't bookmarked