bash loop through string to find first recurring character

gistfile1.txt

#!/bin/bash
#loop through characters in string taking input from first param

string=${1}
for (( i=0; i<${#string}; i++ )); do
  #echo "${string:$i:1}"
  for (( x=(($i + 1)); x<${#string}; x++ )); do
    #echo XXX- "${string:$x:1}"
    if [[ "${string:$i:1}" == "${string:$x:1}" ]]
    then
    echo "${string:$i:1}"
    exit
    fi
  done
done