Declaring a Javascript variable twice in same scope – Is it an issue?

As you said, by twice of more the same var, JavaScript moves that declaration to the top of the scope and then your code would become like this:

var a;
a = 1;
a = 2;

Therefore, it doesn’t give us any error.

This same behaviour occurs to for loops (they doesn’t have a local scope in their headers), so the code below is really common:

for (var i = 0; i < n; i++) {
    // ...
}
for (var i = 0; i < m; i++) {
    // ...
}

That’s why JavaScript gurus like Douglas Crockford suggest programmers to manually move those declarations to the top of the scope:

var i;    // declare here
for (i = 0; i < n; i++) {    // initialize here...
    // ...
}
for (i = 0; i < m; i++) {    // ... and here
    // ...
}