If you are a Aasimar warlock who's otherworldly patron is The Celestial you will know the light cantrip from both race and class. Additional once your reach 6th level in warlock you will have 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?

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 addition 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 the Darkvision feature, 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 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' 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 Ancients Paladin 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.

Undying Sentinel

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 Feature 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.

RAW, you waste features, but talk with your DM.

@Xirema has written up an excellent answer explaining the situation in some detail. As they put it, being an Aasimar Celestial Warlock is highly thematically appropriate, but tactically deficient. On the other hand, that's exactly the sort of situation that often leads to sympathetic DMs. If you want this particular combination for yourself, talk with your DM to see if you can get a houserule in your favor.

If it were my game, for example, I'd let you take another cantrip (which I'd probably pick for thematics) instead of your second light spell, and possibly give you a different thematically appropriate uncommon resist instead of double-stacking that.

Houserules are houserules, and "make sure that super-thematic character builds are also tactically viable" is one of the best reasons to apply them.