I have been having an issue with GST calculating twice on an invoice that has 2 payments receipted t

For example, an invoice was created for $77.00 ($7.00 GST).
P and L is showing:
Payment was received on 02.04.15 for $70.00 (report shows gst as 7.00 and net received as 69.34)
The second payment received on 09.04.15 for the remainder $7.00 (report shows gst as 7.00 and net received as 0.66).

Our business reports on a cash basis quarterly.