Replacing wall oven and service wiring has two reds, a white, and an insulated ground. Why are there two reds instead of one red and one black?

 

Standard color code is red, black, white.

There is no compelling reason why it must be that way.

If wiring is in conduit, electrician just had more red that day, or ran out of black.

White and green and a few other colors are important by code. The two phase conductors are normally interchangeable and may be the same color.

 

Assuming it's 220VAC then the two reds are both hot leads. For 110V you typically have one hot (black) and one neutral (white). In this case you will get 110V from either red to white.

 

If you need to ask that question then you probably shouldn't be messing with it. Some things like 220 are best left to a pro.

 

Standard color code is red, black, white.

There is no compelling reason why it must be that way.

If wiring is in conduit, electrician just had more red that day, or ran out of black.

+1 on possible reason.

Also the code states that if a non-standard colour conductor is used, technically all relevant conductor termination points should be marked by tape indicating the correct or intended colour.
Max.

 

Maybe one of the reds has some black tape on it and you didn't know that's supposed to be a marker.

 

Standard color code is red, black, white.

There is no compelling reason why it must be that way.

Nope, nothing beyond the National Electric Code, the local inspector, and your insurance company.