user exit message only once

Hi

I am currently working on a SD requirement, in VA01 after giving sold to party details on enter it should raise a warning message comparing few conditions.

Yes it is pretty easy, i am using user exit MV45AFZZ and routine USEREXIT_MOVE_FIELD_TO_VBAK.

While testing the warning message is displaying again and again like after enter sold to party, its displaying the warning message.

again if we give any line item again the code is executed and displaying the warning message.

If we miss any header data along with that message first warning message is displayed and then missing data message is displaying.

Can some one tell me how to display this only once.

PS: I can do this using EXPORT and IMPORT a flag to memory, but is there any other better way?

Regards,

NK