I'm going to state an unpopular opinion among those applying and a common practice outside of academia: I wouldn't mention this undergrad degree.

First, unless the admissions folks have enough time and resources to conduct a deep background check or analysis, they can't confirm you have a second undergraduate degree. At best, they might wonder what you were doing in the years you've attended that school if nothing else is listed in your resume in that time frame.

Second, there is no law or legal requirement, at least in the U.S., that applicants must report all of their academic degrees and performance for school applications. It's another scenario if you're applying for financial aid or government grants.

Third, after you're accepted or admitted, no one will care nor have the time and resources to look back at your application file and audit/scrub it for discrepancies beyond transcript verification (of the degree you reported), residency/international compliance, and financial needs assessment. Even if you put this second degree in your CV/resume after being admitted, I doubt anyone who skimmed your application will remember this omission.

If you want a moral ground on this, then since this omitted undergrad degree is not related to the graduate field of study you've applied for, then it's not relevant toward your application profile. Not 100% pure and true, but enough to give you a moral excuse/argument.