Hello Swapnil,
It is always recommended to make use of user-exits for such requirements.
Here is the solution
Put this code in the user-exit IMRC0004 (include ZXMRCU04)
DATA: OLD_READG TYPE IMRG-CNTRR.
SELECT SINGLE MAX( CNTRR ) FROM IMRG INTO OLD_READG WHERE POINT = IMPT_DATA-POINT.
IF IMRG_INS-CNTRR = OLD_READG.
MESSAGE: 'Counter reading equal to previous reading not accepted' TYPE 'E'.
ENDIF.
After this, when user puts reading equal to previous counter reading (IK11), and hits Enter button he gets the standard warning message first
His second hit on Enter button to override the above warning, triggers the desired error message.
The user is free to modify the reading in the same screen and save the document. (Error does not force him to re-run IK11 for correction).
This I have tested and working well. (Hope I need not specially mention that the user-exit above needs to be a part of a project through CMOD).
Best of luck
KJogeswaraRao