How to copy the value from one “column” to another in Bash?

I have a file with lines:

username="user1" email="vova.tupkin@msn.com"

I need to take the username from the email address and replace the username value with it,
so the end result will be:

username="vova.tupkin" email="vova.tupkin@msn.com"

I was thinking in this direction:

#!/bin/bash

while read LINE; do

FUN=$(awk -F """ '{ print $2}')
RUN=$(awk -F """ '{print $4}' | awk -F "@" '{print $1}')


sed 's/$FUN/$RUN/g'

done < test.file