While they appear almost identical in the bottle, the subtle differences become more obvious during application. First, Particulière is lighter than its OPI counterpart. It is also more brown than YDKJ, which is more gray. Finally, the Chanel formula is more opaque and creamier - so the coverage is better than OPI.
If taupe-y nail polishes are a once in a while kind of thing for you, there's no reason you need both. But if you're like me and can't get enough of an offbeat yet neutral nail, I would recommend picking this up - even if you already have YDKJ.