Voucher: Can pay Sales/purchase Receipt multiple times

OpenERP 6.0.1

1. Go to Accounting / Customers / Sales Receipt; enter a receipt (e.g. for $100).
Customer's Total Receivable amount has correctly been increased (e.g. went from $0 to $100).

2. Press the Pay button.
3. Press the Validate button.
4. Press the Save button in the toolbar.

Payment has correctly been taken.
Customer's Total Receivable amount has correctly been decreased (e.g. back to $0).

5. Go back to the Sales Receipt
6. Press the Pay button.
7. Press the Validate button.
8. Press the Save button.

List your customer payments.

There are multiple payments!

Check Customer's Total Receivable - it has been updated AGAIN (e.g. is now -$100 - wrong!).

You should not be able to pay multiple times.