Cita:UI Designer/Artist Benoit Perreault worked for Ubisoft from 2004 to March 2012. On his online portfolio - in the section showing his video game work and assets, he lists the three images I'm posting here.

The disclaimer NOT A GAME is ambiguous. The images could possibly be his own personal work, but I doubt it for the following reasons:

    The images are listed with his industry work, strongly suggesting they were a part of his career.

    The images list a specific format (XBLA, PSN (likely a misspelling of PNS)) - fan-art world have no reason to list a target format.

    He worked for Ubisoft for seven years and now works for Eidos. There is no reason I can fathom that he'd be creating Ubisoft fanart during his new career at Eidos.

    The images are located in a URL subdirectory called "Projects" alongside his other professional work - suggesting the game was a Ubisoft project at one point.

Therefore I think Assassin's Creed Duel is a Ubisoft game that never made it past early development. It was either abandoned while still in its concept stages, or developed and cancelled early before any announcement was made of its existence.

I have never played an Assassin's Creed game yet, so I'm not best placed to scrutinise the screenshots. It does seem to be a 2.5D fighter though.