How do colleges know if you're doing multiple degrees at the same time ?

[TrailRunr]

From a resources or subsidy standpoint it still makes no sense. They do not bar you from attending,

Yes, they will bar you from attending.
Freshman admission profile | UC Admissions

Places like UCLA and UC Berkeley turn down most of their high school applicants. Around 80K - 90K applicants try for the around 15K admit spots. We're talking about admit rates somewhere down in the 15% range. Why should someone who already has a degree take a spot away from a high schooler?

Summer sessions at UC are open access, but they won't let you earn a degree just using summer session courses.