Check To See If Value Previousl Entered.

I would like to check to see if a previous date value has been entered for a specific record and display a message if it has. I have spent some time researching vlookup and vcount but im not quite there in what i want to achieve.

I have a data entry subform, linked to my main form through StairID. The subform is called tblEstateInspection and has a PK called estateinspectionID, stairID is a FK and links it main table.

Like i say i have been investigating what other people have done using dlookup / dcount, what i want to do is check that a specific date has not been entered for another inspection for the same stairID, i guess what i have got below at the moment just looksUp the value and because it is never null the messagebox always shows? How can i ammend it so that if it finds records that have same stairid, checks Inspectiondate field and only shows message if the new date matches old date

Private Sub InspectionDate_AfterUpdate()
Dim strFilter As String
Dim check As Variant
strFilter = Me!StairID

check = DLookup("[inspectiondate]", "tblEstateInspection", "[stairID] =" & strFilter)

If Not IsNull(check) Then
MsgBox "You have entered this exact date for this staircase before, be careful!", vbExclamation
End If
End Sub

Thanks