Duplicate Releases are entries in the database where the same release has been added or imported from FreeDB multiple times.
On the SiteMenu, under Edit the Data -> Suggestions, there is a DuplicateReleasesReport that lists many suspected duplicates.
There are a number of cases where similar releases are not duplicates:
Different releases may have additional (bonus) tracks added.
Different releases may have different versions (longer, shorter, or otherwise changed) of certain tracks.
A release may be present multiple times: both with the original titles and with titles translated or transliterated into (an) other language(s). If these duplicates were never really released but only in the database to have a different representation, they are called VirtualDuplicateReleases.
A release may be re-released as a RemasteredRelease, where the track listing and times are the same, but the music sounds different. (But conversely, if it's a ReReleasedButNotRemasteredRelease, it is ok to enter a MergeReleasesEdit).
This is a fuzzy case; remastering may not change the audio of the track that significantly. As a rule of thumb, if the remastered and original versions share a significant number of TRMs, there is no effective difference between the releases, and they should be considered duplicates. @alex
I don't think it's particularly fuzzy. If the vinyl, minidisc, and cassette releases can be combined with the CD release (that has the same number/order of tracks, then a separate 'remaster' CD is a bit odd. There should maybe be a tickbox for 'remastered' attached to a specific release date, but it's of very little importance in my opinion. -- bawjaws
There are or should be entries describing these under StyleGuidelines and other places (InterNationalization?).