I hope somebody can help

I need to have a message box pop up if somebody enters information in a cell such as C9 in more than one sheet of an Excel file.

Scenario:

a) 5 sheets in file - each the same layout - weeks 1 to 5 for weeks of month
b) each row can only have one occurance in any month - thus only one sheet can be populated
c) columns C,D,F and G are used for data entry of date of month for each week - 1 to 31
d) columns T,U,X, and Y have formulas to test for duplication of data entry

X Column sample
=IF(ISBLANK(G9)=TRUE,"OK",IF(OR(ISBLANK('Cleaning Week 1'!G9)=FALSE,ISBLANK('Cleaning Week 3'!G9)=FALSE,ISBLANK('Cleaning Week 4'!G9)=FALSE,ISBLANK('Cleaning Week 5'!G9)=FALSE),"DUPLICATE ENTRY","OK"))

e) Y Column tests if any duplicate entries exist - if so - result = 1
=IF(OR(T9="DUPLICATE ENTRY",U9="DUPLICATE ENTRY",W9="DUPLICATE ENTRY",X9="DUPLICATE ENTRY"),"1",)

f) Z column tests if column Y = 1
=IF(Y9="1",1,)

g) VB Change Event
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Cells(Target.Column, 26) < 0 Then Run ("Date_Warning")
End Sub

h) Date Warning Msg Box
Sub Date_Warning()

Dim spudd As Variant
spudd = MsgBox("You have entered a date for this unit that is already entered in another week! Please check your entry and correct.")

End Sub

Thanks
Danny