How can two different breakers both turn off the same circuit?

The break circuit must be using stair case schema to turn off the light. It works like a XOR gate in digital logic, the switches should be in opposite position to close the circuit.

You got to short circuit one switch to get rid of this configuration.

That could be a split bus panel, meaning that there are two sets of power distribution buses. It used to be allowed by something called the "6 handle rule" (which allowed up to six of them in a service panel) back when circuit breakers cost a small fortune.

They were wired like this:

And might look like this:
Note the gap in the breaker buses at center where there are no breakers, but you can see heavy black wires connecting to the right end of the secondary buses.

The cables from the meter connect at right (which would conventionally be the top of the panel) and feeds the six double breakers. Five of them feed directly to heavy loads: range, water heater, dryer, baseboard heaters, etc. The top-left double ganged breaker feeds the second bus in the left half of the photo.

In this configuration, any circuit fed from the lower bus would be shut off if either its own breaker were switched off, or the feed breaker—which also shuts off almost everything in the building.

simple,

The circuit has been picked up over the years and its feeding the same circuit and since its on the same phase there is no issue with the breaker tripping. If one of those breakers gets moved one bus up or down it will be a direct 240v short. So leave it as it is or find where it was back fed. Probable happened on a troubleshoot to regain power. Then after some time the old feed came back on because it was probably a loose connection. 99% thats it.