This is an issue that is NOT solved by any Add-on. One Add-on that seems to apply doesn't appear to be trustworthy. It seems that issues like this go on for years with no resolution. This is NOT acceptable. It's like the issue of Tags, and the inability to sort them. That also has been going on for years and has not been addressed, though there was a blurb some months ago that said it was going to be. In the case of Bookmark doubling it appears to be interfering with Tags so some categories of tags do not show up, although checked. When viewing the properties of a Bookmark that is Tagged it has MANY of the same Tags applied to it, which was NOT applied by the User. Not enough interest seems to be being applied to fix these problems. WHY IS THAT? Please provide an application to fix a User's issue of having multiple Bookmarks that are exactly the same. This SHOULDN'T be provided by an Add-on developer, as it is a DEFECT in the FF browser. Answers to this question should NOT request details from this User. Many others are having the same issue, so there is no point in regurgitating what is already known. My time is limited. Just take my word for it that this is happening. I will not post screen shots for that reason.