Quick Answer

Duplicate identifier in same block scope

Understanding the Issue

TypeScript uses block scoping (let/const) which prevents redeclaring the same variable in the same scope. This prevents common bugs from var hoisting.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Redeclaration
let x = 1;
let x = 2; // Error

function demo() {
    const y = 1;
    if (true) {
        const y = 2; // OK - different scope
    }
}

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Unique names
let x1 = 1;
let x2 = 2;

// Solution 2: Different scopes
function demo() {
    {
        const y = 1;
    }
    {
        const y = 2; // OK
    }
}

// Solution 3: Reassign instead
let z = 1;
z = 2; // Allowed for let

Key Takeaways

Use unique names or different scopes to avoid redeclaration errors.