Control of duplicate invoice posting at T.Code FB70

Question

Dear Experts
As per clients need to we need to control with error message when user trying post duplicate customer invoice at T.Code FB70.
we request your help on how do we can meet this requirement.
if any body know the relevant application areas for message control, pleas let me know. 
I tried with application area F5, but it is not working for customer postings.
Thanks in advance.
Best regards,
S. Habib Pasha.

Answer 1

Hi,
I used this validation for vendor invoice.
$$1 is the document number, which is about to be generated after passing through this validation.
So far as the check part is concerned, I used user exit form and coding in program ZRGGBR000, which is a copy of standard program RGGBR000. The coding is as follows-
exits-name  = 'U300'.
  exits-param = c_exit_param_none.        "AP Duplicate Invoice Check.
  exits-title = text-104.                 "Posting check
  APPEND exits.
FORM u300 USING b_result.
*****Local Variables
  DATA:    l_bukrs   LIKE   bkpf-bukrs,  "Company Code
           l_belnr   LIKE   bkpf-belnr,  "Doucment Number
           l_gjahr   LIKE   bkpf-gjahr,  "Fiscal Year
           l_buzei   LIKE   bseg-buzei.  "Number of Line Item
  b_result  =   b_true.
  SELECT  bukrs
          belnr
          gjahr
     FROM bkpf
     INTO (l_bukrs, l_belnr, l_gjahr)
     WHERE bstat = bkpf-bstat
       AND xblnr = bkpf-xblnr
       AND stblg = ' '.
    IF sy-subrc = 0 .
      SELECT  buzei
         FROM bseg
         INTO l_buzei
         UP TO 1 ROWS
         WHERE belnr = l_belnr
           AND gjahr = l_gjahr
           AND bschl = '31'
           AND lifnr = bseg-lifnr.
        IF sy-subrc = 0.
          b_result =  b_false.
          bkpf-belnr = l_belnr.
        ENDIF.
      ENDSELECT.
    ENDIF.                            "closed for SY-SUBRC
  ENDSELECT.
ENDFORM.                                                    " U300
Regards
K Khatri