transformHeader called twice

I have this sample code

let counter = 0;
let csv = "First name,Last name\nJohn,Doe"
let json = Papa.parse(csv, {
    header: true,
    transformHeader: (header, a) => {
        counter++;
        console.log(a);
        return header;
    },
 });
console.log({ counter })

This is the result from running it: It's interesting that the second argument starts as a string and then becomes a number. I think this suggests transformHeader is being executed from different lines each time.

Logging "header" returns this: "First name" "Last name" "First name" "Last name"

It seems like transformHeader is executed twice for the first row. This is causing a bug in my application and I don't know why this behavior is happening.

I'd really appreciate some help with it. Thanks in advance!!