I seem to recall reading (within the last year or so) that an aircraft's CofA is invalidated following refuelling until a fuel sample has been taken and checked.
If this is the case, it stands that fuel samples must be taken after every refuelling.
Maybe the most appropriate time to refuel, wherever practicable, would be at the conclusion of flight A, which would probably give any contaminant time to settle before the next flight; the only a/c movement between refuelling and the next pre-flight check (and flight) would be taxi-ing.
Saying that, how long would it take water contamination at the bottom of the fuel tank to get into the engine and cause a stoppage?
Or is that like asking, 'how deep is a hole?'
Regards,
2close