I want to calculate the sums of certain values that contain matching criteria.

For example:
Ref Inv# Amt
xxy 27 $42.00
xxa 28 $13.50
xxy 29 $35.00

I want to have a fourth column that shows the total value of all invoices
that share a reference number. So in this instance it would end up looking
like this:

Ref Inv# Amt Total
xxy 27 $42.00 $77.00
xxt 28 $13.50 $13.50
xxy 29 $35.00 $77.00

Any suggestions?

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KKD