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:
At 3rd level, you gain darkvision out to a range of 60 feet. If you already have darkvision from your race, its range increases by 30 feet.
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:
Starting at 1st level, you have darkvision with a range of 120 feet.
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:
Relentless Endurance. When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can't use this feature again until you finish a long rest.
And Undying Sentinel, an Oath of the Ancients paladin feature:
Starting at 15th level, when you are reduced to 0 hit points and are not killed outright, you can choose to drop to 1 hit point instead. Once you use this ability, you can't use it again until you finish a long rest.
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:
Damage Resistance and Vulnerability
Some creatures and objects are exceedingly difficult or unusually easy to hurt with certain types of damage.
If a creature or an object has resistance to a damage type, damage of that type is halved against it. If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.
Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance. For example, if a creature has resistance to fire damage as well as resistance to all nonmagical damage, the damage of a nonmagical fire is reduced by half against the creature, not reduced by three-quarters.
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.