Duplicate Entry

[Asif]

I have two tables, tbl_Production and tbl_ProductionAppend. I also
have a form frm_Production which has the following Record Source;

SELECT tbl_PRODUCTION.PRODUCTIONYear, tbl_PRODUCTION.PRODUCTIONMonth,
tbl_PRODUCTION.PRODUCTIONMontlyTotal, tbl_FAMILY.FAMILYType
FROM tbl_FAMILY INNER JOIN tbl_PRODUCTION ON tbl_FAMILY.FAMILYID =
tbl_PRODUCTION.PRODUCTIONFamily;

(tbl_Family categorises the different productions)

I have a combo box on frm_Production of which the row source is

SELECT tbl_MONTH.MONTHID, tbl_MONTH.MONTHName
FROM tbl_MONTH;

Once I select a month and enter the data for each category in
frm_Production an Append query runs where the details from
frm_Production are put into tbl_ProductionAppend.

Now If I close frm_Production and then re-open it and say I select the
same month again I would like a message box to appear telling the user
that data for this month already exists!

Any suggestions

Thanks