I have 2 text fields that are exactly same & want the 1st instance to automatically populate the 2nd

I have another form in LiveCycle Designer ES where 2 text fields that are exactly the same at different parts of my form. It's a common question perhaps, but I need the 2nd instance to automatically populate AS SOON AS the first one is filled in.

Basically I created a checkbox where customer ticks if "shipping same as billing address". What I want is this: As soon as that box is checked, I want those duplicate fields to fill in automatically with the already-filled in shipping info.

Thank you.