Sorry but I just thought of something else - if you are worried about honesty and people purposely billing a company 2x maybe  you should look into your own people who I guess are purposely giving away money.  I've been given as much as 180.00 for an 18.00 order and I have been paid 2x too many times to enumerate.  And in the particular case of the overpayment of 180.00, that credit invoice was still deducted from the over $1,500.00 this company just paid me off from 2 years ago. 

I have had to show companies that they paid the same order with 2 different voucher numbers issued.  These occurrences have occurred far more times than I have ever accidentally billed.  And because credits issued and checks in the mail cross, I have to call the accountants and over the phone issue a credit to outstanding invoices numbers - it's a pain in the ass - but I never accused your employees of wanting to jilt their employer and give their money away.  And in my instance, I remember what happened to my billing now (it happened years ago), my computer kept freezing and I didn't think the invoices saved so when I reset the computer I entered a new invoice.  The invoices that I didn't know existed at the time were never sent until I went through and picked up past due invoices and sent them out as past dues which is when we found out together with the company they were duplicates.  Now the companies that this happened to and I worked with knew this was not purposely done as I was trying to figure out what was happening and it might be a different case in other situations.  Just wanted to explain mine.  My companies know that if they accidentally double pay me there is a phone call to them and I would never take what isn't mine.