Forms :: Only One Person In A Field

I have a form where one person wil insert a new record and click on a button that will display his login name and date she/he inserted the new record. A second person will later check if everything was entered correctly and this person will also press a button that will insert his login name and date in a field.The thing is that the same person can't do the quality check and insert the new record. So if in the field "PersonPassQA" is filled in and the person will do the same in "PersonQA", she/he should get a warning and the field should stay empty. This is the code I use to pass the PersonPassQA:

Code:
Private Sub Command24_Click()
'Date pass to QA and person passing to QA
Me.[DatePassedQA] = Date
Me.[PersonPassQA] = Environ("Username")
Me.Refresh
End Sub

[code]...