Displaying message box in once

[EMILYTAN via AccessMonster.com]

I have a main form with a subform attached to it which is a single form.

In the subform I have this expression :-

strType = Nz(DLookup("WIPType", "Inventory", "PartNumber='" & txtPartNumber &
"'"))
If txtW_Type <> strType Then
MsgBox "The Item Type for Part Number " & txtPartNumber & " is incorrect!
Please refer to the inventory database for the correct type"

This is the similar thing I want...But I want it to appear once and listed
those PartNumber that is not the same type in the inventory when the main
form load....

 

[EMILYTAN via AccessMonster.com]

Appreciate if someone willing to have a look...
Well, struggle for quite a long....seems can't get it...