Now i understand. try this:-

Private Sub Worksheet_Change(ByVal Target As Range)
Dim MyRange As Range
Set MyRange = Worksheets("monday").Range("A1:A100")
If Intersect(Target, Range("A1:A100")) Is Nothing Then Exit Sub
For Each c In MyRange
If c.Value = Target.Value Then
MsgBox ("You entered " & Target.Value & " Previously")
End If
Next
End Sub

As you will note it minitors the range a1 a100 and when a cell changes it
checks if the same data are in A1 - a100 i=on a sheet called Monday. You will
need to expand it for other days of the week.

Mike

"ClaireView" wrote:

Thanks, Mike.

She's entering client numbers, with lots of other data in each row (address,
amount paid, etc.).
When she enters a client number she's already entered that day, I've set
Data Validation to give her an error message ("Hey, goof, you've already
entered this one!") which stops her from entering it a second time.

But each day she starts fresh with a new Worksheet in her Workbook. If a
client came in Monday and Wednesday, she wants to be stopped from entering
him a second time on Wednesday. Hope this is clear. I'd like her to be
stopped as she's entering the data, not comparing lists after the fact. Hope
this is clear.

"Mike H" wrote:

You could look at something like this

http://www.exceltip.com/st/Compare_t...ce l/477.html

which is how to compare 2 worksheets but if you could be more specific in
you question then a more specific answer is likely.

Mike


"ClaireView" wrote:

My co-worker wanted to avoid entering duplicates in her Excel 2002 document.
I was very proud of myself for discovering Data Validation, and that worked
fine. Now the problem is harder:

She has one workbook and every day she adds a new worksheet to it. She
wants to make sure the data she's entering wasn't also entered on a previous
day. This is beyond me. Any ideas?