So I've looked around and can't find an answer that quite fits what I'm wondering. For example, if I were to already have fire resistance, and I get hit with a red dragon's fire breath, would casting absorb elements halve the damage I take(effectively making it 1/4 damage) or not reduce damage at all because I already have 1 source of resistance?
You either have resistance or you do not, different sources of the same resistance type do not stack.
Resistance and vulnerability (P197 of the PHB)
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.
The spell functions as normal, but offers no defensive benefit.
You either have or do not have resistance. Having it from two different sources has no effect, unless some effect specifically ignores resistances provided by one source or not another - I am not aware of such an effect.
That said, casting the spell will still cause you to deal the extra damage, which may be significant.
\$\begingroup\$ It would be good to support this with a quote from the rules, like the one in the other answer \$\endgroup\$ Feb 14, 2021 at 17:24