It's generally not even unmet need. Usually the duck curve hits at times well below the actual annual peak so there's enough generation capacity available.

But some of that capacity is slow to respond and need time to ramp up, which may be longer than the time taken for solar to ramp down.

The simple solution to this is to start those slow to ramp sources a little earlier and throw away their excess energy output.

Obviously that's wasteful, but as long as the generation mix is overall cheaper and less carbon intensive with solar added, which it is, then it's not a big deal. And obviously as soon as you add storage then they can help these sources ramp more efficently at first and expand to replace them entirely over time.

All in all it's one of the least problematic problems that ever got its own name.