Validation for duplicate AP invoice

Question

We would like to prevent duplicate AP invoices from being posted through FB60.   We created an FI validation in OB28 in which the prerequisite = Tcode FB60, and the check is a user exit to check the values in vendor / company code / reference fields and if those 3 fields have the same values as in an existing document, to generate an error message preventing the duplicate document from being posted.
The above works well for creating new invoices.   However, when we have to make changes to an existing invoice (created through FB60), the system also calls on that validation and generates the error message, even though the only fields we can change on the invoice is payment terms, payment method, payment method supplement and text. 
Other than modifying the user exit, does anybody know of a better way to do this validation or to prevent duplicate manual invoice from being created?  
thank you.

Answer 1

This has already been answered before so extracted part of the reply
there are two duplicate checks in SAP:
1. the one for FI documents, this is relevant for all F... postings (but nor for MIRO!)
this can be turned off or on (in vendor master record) and 6 fields are checked:
Check Flag for Double Invoices or Credit Memos
Indicator which means that incoming invoices and credit memos are checked for double entries at the time of entry.
Use
Checking Logistics documents
Firstly, the system checks whether the invoice documents have already been entered in the Logistics invoice verification; the system checks invoices that are incorrect, or invoices that were entered for invoice verification in the background.
Checking FI documents
The system then checks whether there are FI or Accounting documents that were created with the original invoice verification or the Logistics verification, and where the relevant criteria are the same.
Checking Logistics documents
In checking for duplicate invoices, the system compares the following characteristics by default:
Vendor
Currency
Company code
Gross amount of the invoice
Reference document number
Invoice document date
If all of these characteristics are the same, the system issues a message that you can customize.
When you enter credit memos or subsequent adjustments, the system does not check for duplicate invoices.
Exception: Country-specific solution for Argentina, where invoices and credit memos are checked for duplicate documents.
No message is issued if you enter a document that has previously been reversed.
Dependencies
The system only checks for duplicate invoices in Materials Management if you enter the reference document number upon entering the invoice.
In Customizing for the Logistics invoice verification, you can specify that the following characteristics should not be checked:
Reference document number
Invoice document date
Company code
This means that you can increase the likelihood that the system will find a duplicate invoice, because you can reduce the number of characteristics checked.
Example
The following document has already been entered and posted:
Reference document number: 333
Invoice date: 04/28/00
Gross invoice amount: 100.00
Currency: EUR
Vendor: Spencer
Company code: Munich
You have made the following settings in Customizing:
The field "Reference document number" and "Company code" are deselected, which means that these characteristics will not be checked.
Now you enter the following document:
Reference document number: 334
Invoice date: 04/28/00
Gross invoice amount: 100.00
Currency: EUR
Vendor: Spencer
Company code: Berlin
Result
Because you entered a reference document when you entered the invoice, the system checks for duplicate invoices.
The reference document number and the company code are different from the invoice entered earlier, but these characteristics are not checked due to the settings you have made in Customizing.
All other characteristics are the same. As a result, the system issues a message that a duplicate entry has been made.
If the "Reference document number" had been selected in Customizing, the system would have checked the document and discovered that it was different from the invoice entered earlier, so it would not have issued a message.
Checking FI documents
Depending on the entry in the field "Reference", one of the following checks is carried out:
1. If a reference number was specified in the sequential invoice/credit memo, the system checks whether an invoice/credit memo has been posted where all the following attributes agree:
Company code
Vendor
Currency
Document date
Reference number
2. If no reference number was specified in the sequential invoice/credit memo, the system checks whether an invoice/credit memo has been posted where all the following attributes agree:
Company code
Vendor
Currency
Document date
Amount in document currency
2. the one for LIV.
this can be customized (as stated above) and is relevant for MIRO (and not for F... postings!).
Company, Reference and Invoice date
Set Check for Duplicate Invoices
In this step, you can configure for each company code if the system is to check for duplicate invoices when you enter invoices.
This check should prevent incoming invoices being accidentally entered and paid more than once.
You can choose whether to activate or deactivate the check criteria of company code, reference document number and invoice date for each company code. The more criteria that you activate, the lower the probability of the system finding a duplicate invoice. The Accounting documents are checked first, followed by documents from Logistics Invoice Verification (only incorrect invoices or those entered for verification in the background).
When checking duplicate invoices, the system compares the following
attributes in the standard system:
Vendor
Currency Company code
Gross invoice amount Reference document number
1. Invoice date If the system finds an invoice that matches all attributes, the system
displays a customizable message.
If you are entering credit memos, subsequent debits, or subsequent
credits, the system does not check for duplicate invoices.
The exception is the Argentina country version, where the system checks
for duplicate invoices and credit memos.
If a previously processed document is later cancelled and then entered
again, no message is displayed.
Requirements
The system only checks for duplicate invoices in Materials Management
if you specify a reference document number when entering the invoice.
In Customizing (IMG) for Invoice Verification, you can specify that the
system check the following attributes