A Duplicate Sue is not the same as a Doppleganger!Sue or a Replacement!Sue. A Duplicate Sue is a Mary Sue or Gary Stu that is not trying to pretend to be a canon character, but a Sue/Stu that merely emulates them in some way.
For some reason, a lot of these Sues copy the appearance of a canon character, but use the author's name or a self-insert name.
There are a few different kinds of this Sue:
Same Canon DuplicateEdit
This is a character that looks or acts almost exactly the same as another canon character within the same canon. This may be a Uncanonical Relative, a clone, a copy, or may not even try to excuse or explain it's existence. Either way, this character steals the appearance of another character and possibly their powers. Expect them to be 'Just like X, but better!'
Different Canon DuplicateEdit
This is a character that looks or acts almost exactly the same as another canon character from a different canon that the fic is taking place in. Instead of crossing over canons, badficcers may simply copy a character or rip off their abilities and give it to an OC in another canon. It may take a little bit of research to spot these.
Although an unfortunate OC rather than a Stu, Agent Derik began his existence as one of these: a Phantom of the Opera look-alike on Pern. The author didn't even change his name apart from sticking an apostrophe in it due to his being a dragonrider.
More of an art fad than a writing fad, but still applicable, a character recolor is a character that looks exactly the same as a canon character... except for their color scheme. For example, they change the color of the hair and the clothes, and perhaps add a few tattoos, glasses, or some other small feature to make them somehow different from the character they're copying.