iApp Presentation section Optional questions

If have a list of six choices available with one to be selected. Based on the selection, I need to modify the question that follows. Logically, I want to do what has been commented out below which gives me the error message, “Redefined symbol (enc)”. I realize in this example I am presenting only a single choice which cannot be changed however the same logic would apply if multiple choices were presented.

Is there a way to reuse the “enc” variable below or do I need to create enc-1, enc-2, etc. and then apply some logic to determine which one has an actual value?

section ike_phase2 {
    choice auth_algo default "aes-gcm128" {
        "SHA-256" => "sha256",
        "SHA-384" => "sha384",
        "SHA-512" => "sha512",
        "AES-GCM128" => "aes-gcm128",
        "AES-GCM192" => "aes-gcm192",
        "AES-GCM256" => "aes-gcm256"
    }

    optional ( auth_algo == "sha256" || auth_algo == "sha384" || auth_algo == "sha512" ) {
        choice enc {
            "SHA-256" => "sha256"
        }

   optional ( auth_algo == "aes-gcm128" ) {
       choice enc {
           "AES-GCM128" => " aes-gcm128"
       }

   optional ( auth_algo == "aes-gcm192" ) {
       choice enc {
           "AES-GCM192" => " aes-gcm192"
       }

    }
}