I think functions like this should be allowed:

fn f(a: i32) -> bool {
    let a = true;
    return a;
}

However, the spec says:

Two declarations in the same WGSL source program must not simultaneously:

• introduce the same identifier name, and

• have the same end-of-scope.

"§9.1. Declaring a User-defined Function" says:

The identifier is in scope until the end of the function.

"§6. Variable and Value Declarations" says:

The name is available for use in the statement immediately after its declaration until the end of the brace-delimited list of statements immediately enclosing the declaration.

These seem the same to me. They're definitely not clearly different.

I think this is uncontroversial, but here's why I think the code at top should be permitted:

• Shadowing is definitely permitted in inner blocks, so it seems consistent to permit it at the top-level block too.
• Such shadowing is permitted in many popular languages.