4Greens, remember we were at the fore front of aviation where the only way to prove a design was to build and test. The USA is still able to afford fly-off competitions.
In UK the Canberra, IIRC, was unique but the Sperrin and Valiant were insurance policies against failure to meet the full OR for the Victor and Vulcan. The 3 Vs were really therefore intended to be 2 however Vickers tried to stay in the game with the Mark 2.
Other multiple buys were the Swift and Hunter. Sea Vixen and Javelin; argueable de haviland got it right and Gloucester, after 9 attempts was still trying. I doubt you could have got a Javelin off a carrier (more than once).
By the time we got to the Tornado it was our second attempt at a one-size fits all.