Mapping logic -counter UDF required

Hi Experts,

I have below  source/target test XML structures, i need to add -1 counter value increment by -1 in target  if Order ID value is same ,tried using counter but no luck.

<Order>

  <OrderID>AZ1234</Order>

  <Location>CA</Location>

<Region>NA</Region>

<Name>Mike</Name>

<Status>New</Status>

<Card>yes</Card>

</Order>

<Order>

  <OrderID>AZ1234</Order>

  <Location>CA</Location>

<Region>NA</Region>

<Name>Mike</Name>

<Status>Old</Status>

<Card>yes</Card>

</Order>

<Order>

  <OrderID>AZ9999</Order>

  <Location>CA</Location>

<Region>NA</Region>

<Name>MikeSwiney</Name>

<Status>New</Status>

<Card>yes</Card>

</Order>

Output

<FinalOrder>

  <OrderID>AZ1234-1</</Order>

  <Name>Mike</Name>

<FinalOrder>

<FinalOrder>

  <OrderID>AZ1234-2</</Order>

  <Name>Mike</Name>

<FinalOrder>

<FinalOrder>

  <OrderID>AZ9999-1</Order>

  <Name>MikeSwiney</Name>

<FinalOrder>

Thank you so much.