I would be so scared to be a recreational user of MDMA at this point in time, because from what I hear there is a large amount of fentanyl and other concentrated opiates floating around in the street drug supply.
This is one of the great things about the dark web marketplaces - you can order from a supplier who already has a track record of positive reviews, and every incentive to keep it that way.
You can (and should) get your stuff tested.
The effects of MDMA and opiates are entirely opposite - why would someone cut MDMA with fentanyl?
My guess would be it's an issue with cross contamination, not an intentional mixture of the two.