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

Have you checked the new redemption rates for Mileage Plan which go into effect next month March? Does AS collect carrier surcharges for the particular airline you plan to fly? Have you compared the cancellation fees charged by BA versus QR, which now also uses Avios as its redemption currency? (Avios are freely transferrable between BA, IB, and QR.)

I would expect that an award cancellation would be done pretty quickly, but your expectation that you will find two more Business Class award seats at T-331 after you have already booked two at T-355 might be unrealistic. And there is also no guarantee that the two award seats you cancel will go back into award inventory immediately -- or ever -- so do not count on that.

While just about anything is possible, if you are able to snag the two Business Class award seats at T-355, I would not count on being able to secure possibly cheaper award seats at T-331.

Good luck!

Thanks, good thoughts.
As they say, a bird in the hand is worth two in the bush!
For the flight I'm thinking of, there are still 3 left for that date on BA (I do have two)
Out a couple of weeks now from using the AS option
I think AS has very low cancellation fees, but will check, BA definitely does
I read somewhere that many or all reservation systems would block if it receives the same name/DOB for the same flight/class/date, as bookings are filed by name. Looking to see if someone can confirm this
This would be before a change in redemption rates
Jeff