sql sum - dealing with dirty duplicates in join table

Question

I am pulling reports out of a bookkeeping system. The system has a nasty habit of putting duplicates in the DB which it handles internally (instead of making it clean in the first place!)

as an example - this is the totals table for invoice 125:

+------------+-----------+----------+
| invoice_id | code      | amount   |
+------------+-----------+----------+
|        125 | sub_total | 300.0000 |
|        125 | tax       |  30.0000 |
|        125 | total     | 330.0000 |
|        125 | sub_total | 300.0000 |
|        125 | tax       |  30.0000 |
|        125 | total     | 330.0000 |
+------------+-----------+----------+

and the invoice table for the same id

+-----+----------+
| id  | amount   |
+-----+----------+
| 125 | 330.0000 |
+-----+----------+

I would like to generate a total sales and taxes for a period (for Australian BAS)

my MWE query (that works if the data is clean) is

select sum(a.amount) as total_sales, sum(c.amount) as total_GST
from 7cn_invoices a 
    INNER JOIN 7cn_invoice_totals c ON a.id = c.invoice_id
where c.code = 'tax';

However, since there are duplicates in the total table I get total sales double what they should have been. What is the best way to solve this (other than patching the code)?