just curious, am i receiving someone else dirty coins after the mixing?
You are not, the addresses you receive from are funded once and once only I believe and when you ask for the withdrawal they are sent to you, the addresses are pre-funded so I doubt you are getting anyone else bitcoins.
He's asking if the coins he receives could have been used before by someone who has more to hide than himself. The answer is of course yes. Bitcoins are not newly created by ChipMixer, and your Bitcoins are not destroyed by ChipMixer after receiving them. But that's no different than when you withdraw cash from an ATM, or even receive money on your bank: money transfers hands a lot, and whoever owned it before you has nothing to do with you anymore.
Example:
Every British bank note is contaminated by cocaine within weeks of entering circulation