Can I get back into the US with an expired US passport and a valid UK passport? [duplicate]

Quoting from the U.S. DEPARTMENT of STATE - BUREAU of CONSULAR AFFAIRS:

U.S. nationals, including dual nationals, must use a U.S. passport to enter and leave the United States.

Also quoting from the U.S. DEPARTMENT of STATE:

U.S. citizens currently overseas whose passports expired on or after January 1, 2020, may be able to use their expired U.S. passport for direct return travel to the United States until December 31, 2021. Certain criteria apply, and we encourage U.S. citizens to confirm their eligibility for traveling on an expired passport at COVID-19 Traveler Information prior to finalizing travel arrangements.

So if your passport has expired after Jan 1st, 2020, you might be eligible to use your US passport, even if it's expired. If your passport expired before then, you're going to need to go to the US embassy to renew it. Or at least that's what U.S. law requires you to do.

The fact that you might be able to get in with the UK passport is another story.

If you can't use your US passport, because it expired before Jan 1st 2020, I strongly suggest you to contact the US Embassy and ask them for informations. They surely have the most up to date informations, and there might be some additional option for you to use your UK passport for reentry.