Duplicate case values allowed in switch

[tahadostifam]

Duplicate cases should be disallowed. Current code compiles without error:

switch (text) {
    case "Cyrus!" => { printf("Cyrus The Great\n"); }
    case "Cyrus!" => { printf("Cyrus The Great\n"); }
    default => { printf("unknown\n"); }
}

Add validation to prevent duplicate cases.