Not sure I get the situation.

If I am correct, you are not really replacing anything, but you are simp0ly using the space that a spa breaker previously occupied?

Where are you getting two blacks? In one cable? That can't be right.

Forget what was there and tell us what you are doing new.

Can you explain a little better?

Originally there were two blacks and a green and white. The two blacks are attached to the double breaker. So there were four wires running through the conduit to the spa.

Replaced the original wires with 12 gauge through conduit. This is still going outside to a GFI receptacle and then being routed to post lights. So now I have (3 separate wires) a black, white, and green. Changed the breaker to a 20 amp and attached the black to the breaker. The green and white were installed in the exact same place as the original green and white wires.

And PeteyRacer thanks for your response.

If this is a subpanel, the greens (grounds) and whites (neutrals) cannot be on the same buss bar. If this is a main panel, with integral main disconnect, they can be on the same buss bar, but I don't believe they may be attached in the same spot.

Thanks Randy for your response. They are on a main panel but attached one on top of the other on the bar, in a different slot. They are not in the exact same spot. Just changed each wire at the same place. So it sounds like it's good to go.

Odd that the electrician who installed the spa would have pulled two blacks for the hots. The right way would have been one black and one of some other "hot" color (red, blue, etc.).

There's nothing wrong with 2 black wires for the hots. I don't find this odd at all. It's easier just to stock the single color and fully code compliant. In the case of a 240/120 circuit, the 2 hots are interchangeable, so I don't see any advantage to an extra color.