May seem like an odd question, but I'm thinking of flights available on both BA Avios Award and also Alaska Mileage Plan - for two passengers in Business Class.
So airlines like JAL, Cathay and Fiji Air can be available on both Avios and Mileage Plan, but are available several weeks early on Avios.
So booking on Avios would guarantee a seat(assuming released in the first place), booking on Mileage Plab would be cheaper in miles (at risk of award seats being gone)

So if I already had a booking for two people with BA Avios, and then tried to book the same flights with Alaska (before canceling the BA), would the booking systems notice the duplicate booking and block it? Or let it go through?

The other option would be to cancel first, but if it takes time to go through, might be no seats left

Any advice appreciated
Jeff