There are "vote centers" where people can receive assistance with voting. https://kingcounty.gov/en/dept/elections/how-to-vote/ballots/return-my-ballot/vote-centers "Vote centers are available to register new voters, update current voter records, obtain a voter registration card and to provide assistance to voters who need help completing their ballot. Trained staff and specialized equipment are available to help voters with disabilities cast a private, independent ballot." I'm guessing you could also go there if, for example, your spouse had taken your mail-in ballot away and forged it. (You're always allowed to cast a provisional ballot in the event of some complication, too. If your other ballot then turns up it will be flagged and not counted.)

There are also some other accommodations available, such as large-print ballots, a voice recording of the voter information pamphlet, that kind of thing.