const already declared in ES6 switch block [duplicate]

You can create scope blocks around your cases, and the compiler will be happy:

switch (1) {
    case 1: {
        // notice these extra curly braces
        const foo = 1;
        break;
    }
    case 2: {
        const foo = 2;
        break;
    }
}

Read the answer from Igor if you need more context.