I was sold a tandem breaker. It looks like I needed a double-pole circuit breaker for my range top/oven. The tandem breaker shows 120 volts at each line, but together there is zero voltage, instead of 240 volts. How did the old tandem breaker power my oven for all those many years?
Because your panel is a GE Qline
The original breakers was their 1" wide 2-pole breaker. The type straddles two spaces, leaving 1/2" above and below.
You took it to the store, I am guessing a big box, and looked for breakers like that. They didn't have GE 1" 2-poles. But you or they mistook it for a duplex.
GE does not make duplex breakers.
But you found a BR, HOM or Murray duplex. Close enough. The clerk swore it would fit. He "knows" all these types cross-fit (wrong).
You got it home. It wouldn't fit directly, because it couldn't clip on to those little Qline cruciforms on the bus bars.but did fit if you moved it to a different location.
Moral of the story is don't use alien breakers.