In Zhu's example there are two separate Vendors, which is allowed.
In your question, you are NOT allowed to post the same Vendor Invoice No. multiple times for the SAME Vendor. This is a base rule in BC. The concept is when you hit Post you are recording a copy of the invoice received. You should never have a situation where a Vendor sends you the same Invoice No. billing for new lines. Otherwise, when you say you paid a specific Invoice No. they wouldn't know which one it actually applied too.
If your Vendor IS re-using their Invoice No. you can simply add something to the end of the invoice no. (ex. -a or -1) to denote the same invoice used multiple times.
IF your Vendor IS re-using their Invoice No., please pass them my contact information and I'll show them a wonderful accounting system which can automatically create new Invoice Numbers for their billings.
