California's Senate elections

When Kamala Harris became Vice President, that left her Senate seat empty. Under California law, the Governor appoints a person to serve the remainder of an empty seat, until the next general election. So, the next general election, even though the Senate seat was filled almost two years ago, isn’t until this November. So there is an election to decide if Alex Padilla should still be the Senator, or if somebody else will fill the seat. The primary is next week, and the general election to decide which of the two biggest vote-receiving candidates will finish out Harris’s term will be in November.
But Harris’s term ends in January, so the person who wins the election will only fill the seat from November until January.
So there will also be another election next week and in November to start a new term in January. Alex Padilla is also running to serve the new term which starts in January, along with several other candidates.
So we have two Senate elections with the same candidates.

Yes. This is more common in the House, where a vacancy can only be filled by election. It varies by state, but if an incumbent dies/resigns with only a few months remaining in his or her term there will sometimes be a special election for the remainder of the current term held concurrently with the general election for the next term.