I'm sick of this stupid argument so I basically wrote this essay to clear things up.
Chamber is not a duelist - he is a sentinel, he simply does his job in a way that is contrary to other sentinels.
All 4 sentinels, (Chamber, KJ, Cypher and Sage) are incredibly strong defensive picks, that can stop pushes solo, and are incredibly powerful when guarding their territory, they struggle; however, when adventuring outside of their territory (Cypher/Killjoy leaving setup, Chamber when leaving TP radius and Sage when having to play offensively (all of her utility struggles when attempting to take space). This is consistent across all sentinels, and is unique to them (and Viper, who is a Controller/Sentinel hybrid as stated by riot themselves).
Where people often get confused with Chamber is his proficiency in getting kills, especially first bloods. While this was historically not a sentinel thing to do (prior to his release), it is not reason to see Chamber as a duelist. Duelists, as stated by riot, "seek out engagements first". Chamber (by himself) has no utility to seek out engagements. No flashes, drones, information gathering, or mobility. His only "mobility" tool is his roundezvous, which can only allow him to retreat. (no possibility of entrying, unlike a Jett, Neon, Raze, Yoru, Reyna or Phoenix, who all have some sort of tool to engage a fight and break through into space. His other abilities, while kill oriented, are designed that way because to do his job (lock down space), he needs to get kills. Unlike Cypher, KJ and Sage, who can shut down an entire push without getting a single kill, Chamber's kit is designed that he must be taking engagements and getting kills to achieve the same site holding potential. His rondezvous, headhunter, and tour de force are all designed to allow him to 1. take an early fight to get info/pick, like a camera or turret. 2. slow down attackers, like a slorb, swarm grenade or cyber cage and 3. incentivize chamber to play within his territory, like a barrier orb, KJ passive, or trapwire.
to clear up even more confusion, and to hopefully stop any future arguments about agent roles; there is a simple way you can tell what an agent's role is by the way they interact with SPACE
Controllers define the space (split up the map, what parts of the map do you want to claim? what parts do you want the opponent to stay in?)
Initiators weaken the space (study and observe the space you want to weaken, without entering it. weaken the defenders of the space with utility to create openings for attack)
Duelists invade the space (enter the space as fast as possible and during the window of highest threat. quickly claim space for your team to allow them to follow you in)
Sentinels defend the space (hold down and weaken attackers when they attempt to claim your space, retain information on space your team already owns)
As you can see, there are lots of different ways to, define, weaken, invade and defend space, but at the end of the day, the agents in those roles will be best suited for doing that job, even if it may be in a much more unique style than other agents of that class.
TLDR; Chamber whole kit about defending his territory, no initiation tools (unlike duelists), kills/FB =/= duelist.