ORA-04091 table is mutating

Error:

Records Sucessfully Inserted. But when i tried to Update the same field with another PAN Number raising below error.

Cause:
A trigger or a user-defined PL/SQL function that is referenced in the statement attempted to query or modify a table that was in the middle of being modified by the statement that fired the trigger.

Solution:
Rewrite the trigger (or function) so it does not read that table.

Crated Trigger

CREATE OR REPLACE TRIGGER APPS.DUPLICATE_PAN

   BEFORE INSERT OR UPDATE

   ON JAI_AP_TDS_VENDOR_HDRS

   FOR EACH ROW

DECLARE

   A   NUMBER (10);

BEGIN

   SELECT COUNT (*)

     INTO A

     FROM JAI_AP_TDS_VENDOR_HDRS

    WHERE     PAN_NO = :NEW.PAN_NO

          --AND VENDOR_ID = :NEW.VENDOR_ID

          AND VENDOR_SITE_ID = :NEW.VENDOR_SITE_ID

          AND TDS_VENDOR_TYPE_LOOKUP_CODE = 'OTHERS-IND';

   IF A > 0

   THEN

      RAISE_APPLICATION_ERROR (-20454, 'PAN NUMBER ALREADY EXISTS');

   END IF;

END;