Also there is an unwrtten rule that you don't bump each other spies off and I think that goes for execution too.
Look at the Cambridge five. I suppose under treason laws they could have been shot but sometimes it is worth letting them go just to see who they go to.
I think that sometimes they might let a spy continue his spying to see who their contact is they just let him see wrong information that he and the enemy whoever it is thinks is right.
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