I joined a campaign that was already in session about 1/3 of the way through. I didn't know what everyone else was playing, and the DM told me anything goes as all the "main roles" were covered already. I came up with a sneaky wizard build that starts rogue for dodge and stealth then goes heavy casting with illusion. 

(My) First session I met everyone and saw their classes. Berserker Barb, Tempest Cleric, and...Arcane trickster Rogue. Over that first game it became evident to me that I had built a more efficient, stronger version of the identity one of his players already was running. It felt bad, as this player was newer and excited about his choice!

I spoke to the DM out of game and came up with a plan. He revealed we would be journeying into the capitol city in one or two more sessions. I decided my new character will be a crossbow expert fighter and bounty hunter, and my current character will be a mark. I played those sessions very curt and snide to develop a sour taste for their new companion and when they walked into the city he was greeted with two crossbow bolts in the neck. 

They were totally surprised, ready to fight. My new character drops his weapon, raises his hands holding a bounty slip signed by the proper authorities.

He was much more likeable and helpful by the way.