If you are an Aasimar warlock whose otherworldly patron is The Celestial, you will know the light cantrip from both race and class. Additionally, once you reach 6th level in warlock you will have resistance to radiant damage from both race and class.
Is there any ruling on what happens when you have overlapping class and race features?
Best Answer
For your specific example, you simply gain the Light cantrip, there's no interaction as a result of the overlap. You wouldn't gain an additional cantrip of your choice, or any other effect.
For other feature interactions, it depends on whether the feature in question actually says anything about its interaction with other features. For example, Gloom Stalker rangers (found in Xanathar's Guide to Everything) have a feature called Umbral Sight, which grants darkvision, but also has an additional effect:
This is one notable exception to the rule I specified above: for this feature, already having darkvision means that this feature improves it, rather than simply overlapping with it.
Conversely, however, the Shadow Magic sorcerer (also found in Xanathar's Guide to Everything) only says this about granting darkvision:
This has no mention of darkvision from any other source, so for you, you'd only gain the darkvision out to 120 feet, regardless of whether your Race (or a spell effect) grants darkvision.
Note however that, in general, the strongest/longest duration feature generally wins out, so if your race were only giving 60 feet of darkvision, this would expand the range to 120', making it not totally redundant.
Additionally, certain features can be expected to overlap even if they do the same thing. Consider Relentless Endurance, a half-orc racial feature:
And Undying Sentinel, an Oath of the Ancients paladin feature:
These two features can be separately used, meaning a character with both of these features can drop to 1 hit point twice per long rest.
This also applies to racial/class features which permit once-per-rest spellcasting: logically, if both a racial feature and a class feature permit you to cast a spell once per rest, then you'd gain two uses per rest.
Your issue is that, as a cantrip, Light is an at-will spell anyways, meaning there's no benefit to gaining the ability to cast it from a different source. The one noteworthy exception is if some kind of effect associated with the Light spell depends on your spellcasting modifier, and to my knowledge, there are no such effects. If such an effect takes place, and you were to gain the Light spell through being a cleric or wizard instead, then specifying which feature you're using (the racial trait as an aasimar, or the cantrip learned as part of your class features) will matter.
As for your resistance to radiant damage, the PHB is pretty clear about resistance not stacking from multiple sources:
In short, your choice to be an aasimar Celestial warlock is highly thematically appropriate and well-suited to the character from an RP perspective.
It does not, however, give you a lot of tactical synergies.