Yes, they do.
What dealers will do is prepare their individual doses of heroin in wraps with a specific stamp on them so that the junkies know what dealer it comes from.
And then they will put drops of fentanyl in one or two of the doses to intentionally cause a overdose, or even a death, because when word of an overdose or death reaches other junkies, they want that particular brand particularly, because they believe it’s more potent.
You watch too many movies...