Odd double invoice

[Luke <..hidden..>]

In 1.2.18, I posted an invoice.

When going to add a receipt a few days later, I found that the invoice 
appeared twice.  Same invoice number, same amount, exact same content as 
far as I could tell.
The only thing strange about this invoice, was that the third line item 
was a $20 service with a negative quantity (-1).  Again, it appeared 
correctly on both invoices.
When looking at the report, I could see that they had different IDs 
(10665 (or something similar), and 10771).
Could the negative item have caused this in some way?  I have not seen 
such behavior previously.
I was able to delete the invoice (for services only), and only a single 
copy remained.
Luke