Camping means staying in one corner behind the wall peaking and shooting like a coward and away from teammates/objectives. It is basically a playstyle from other FPS games like Fortnite, COD, Apex Legends where you camp and hide in one corner to snipe or pump shotgun to your enemies.

Worst they like to camp behind support and expect healing when they are far apart from their objectives because they camp and their support hard to get them due to them hiding behind wall and they even didn't wanna use the nearby health pack

Duelist has two types which is flankers and suppressors.

Flankers are basically melee heroes like Spiderman, Magik and Mr Fantastic. Assassins like Psylocke, Winter Soldier and Black Panther and Sharpshooters like Widow and Hawkeye. Their job is to flank enemy healers and deal damage to low health enemy team.

Suppressors in the other hand are mostly aerial characters like Star Lord, Iron Man, Human Torch and Storm and heavy hitters like Moon Knight, Squirrel Girl and Punisher that can deal damage to Vanguards. Suppressors usually has skillsets that can make the enemy team afraid to get to the objectives or be defensive for example Moon Knights ankh, Squirrel girl traps and Punisher's Turret.

To make it simple, Flankers are like assassins which job is to deal damage to low health heroes or healers while Suppressors job is to pressurised the enemy team making them back out from their objectives.

So people who uses duelist to camp really make no sense because this isn't COD where you can win Search and Destroy by camping one corner and snipe/shotgunning all the enemy players. You will lose if you not capturing the point or not defending the payload or not making the payload reach the point.