Steve Swatton has been campaigning for years for the release of a "notorious" prison inmate Charles Bronson. Now that he would like to visit the bank robber Swatton is being prohibited by prison officials who may believe that he would swap places with the prison inmate. Which begs the questions: A. Why would Swatton want to swap places with a prison inmate? B. What kind of maximum security prison would make it possible for Swatton and Bronson to swap in the first place? Are they that easily confused by bald men with mustaches?
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Security is about finding a solution that has the smallest chance of failure with the smallest reasonable cost in terms of resources and effort.. not taking unnecessary chances to show how good your security is. It takes compelling reasons to do anything that increases the chance of failure or raises the cost.
The current baseline for security is that the lookalike is nowhere near the prison. Bringing the lookalike onto prison grounds would create additional risk and impose additional cost. There's no compelling reason to do either, especially when saying "no" to the visit is cheap, easy, and preserves the baseline.