There are two slightly different rules when it comes to overlapping game features, one from the PHB (post-errata) and one from the DMG (post-errata):
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect – such as the highest bonus – from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap.
The current answer to my question “Do the Stone Golem's Slow feature and the Slow spell combine?” states the following:
[…] Backed by the PHB’s mention of effect-based potency, I compare each individual effect two features of the same name have and then pick the strongest for each. That also constitutes that the unique parts of one feature are compared against “nothing”, meaning by virtue of existing they’d trump the lack of any counterpart in their twin […]
And goes on to apply this method to blindness/deafness allowing both effects to persist simultaneously in direct contrast to the current answer to the following: “If you cast Blindness/Deafness on the same creature twice, what conditions are applied?”
Which method of comparing two features is correct:
One feature/spell is determined to be “more potent” and all effects of the other feature/spell are ignored.
The effects are compared one-by-one, thus different parts of each feature/spell can be active at once.