I recently came across a newspaper clipping about my Grandfather having completed his second tour as an air gunner (rear gunner) in WW2. My original understanding due to such stories like the Memphis Belle is that this was extremely uncommon. I know that his first tour was based at RAF Heliopolis/Almaza with a Wellington Squadron (trying to identify which one). His second was with 431 'Iroquois' Squadron RCAF based at RAF Burn and RAF Croft.
The odds were against him; the first tour for RAF bomber crew was (at least) 30 sorties, the second was 20. Based on raw probabilities, if average losses were 1.5% per mission less than half of crews (47%) would make it through two tours. During 1943, with German defences at their strongest, the average loss rate for Bomber Command crews based in the UK was 4.5%, giving a 25% chance of surviving one tour and a 10% chance to also make it through a second tour.
Being based in the Middle East improved his odds; Alun Granfield's Bombers Over Sand And Snow covers the operations of No. 205 Group who flew Wellingtons in the Mediterranean, their overall loss rate was 1.9% giving a 56% chance of completing a 30-sortie tour. As Germany's defences weakened, Bomber Command's losses gradually fell - from April to August 1944 they averaged around 2%, from September 1944 to the end of the war they were around 1%. According to https://www.rcafassociation.ca/heritage/history/rcaf-and-the-crucible-of-war/431-squadron/ , 431 Squadron lost 72 aircraft on 2,584 sorties, an average rate of 2.8%, which I believe works out at around a 57% chance of surviving a 20-sortie second tour.