编写一个for循环以查找从活动行到最后一个工作表末尾的值

Question

I have a workbook which contains 4 worksheets, each for one quarter in 2019.
I would like to write a function to check, whether the input of active cell is a duplicate from the active cell row to the end of the last worksheet.

Function checkDuplicate(ByVal ChangedCell As Range) As Boolean
     TelNo = ChangedCell.Value

     Range(ActiveCell.Address).Name = "StartCell"
     For i = ActiveSheet.Index To ActiveWorkbook.Worksheets.count
        Worksheets(i).Activate
        With Worksheets(i)
            Set rng = Cells.Find(What:=TelNo, LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False)
        End With
Next i
Application.Goto "StartCell"
If rng Is Nothing Then
    checkDuplicate = False
Else
    checkDuplicate = True
End If

However, it seems that it cannot even find the duplicated values in the row below the active cell.

Answer 1

As it stands your loop is useless. You could do this with the exact same result: Set rng = Worksheets(4).Cells.Find(What:=TelNo, LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False)

Try this:

Function checkDuplicate(ByVal ChangedCell As Range) As Boolean
  TelNo = ChangedCell.Value

  Range(ActiveCell.Address).Name = "StartCell"
  Set rng = ActiveSheet.Cells.Find(What:=TelNo, After:=changedCell LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False)
  If Not rng Is Nothing And rng.Address <> ChangedCell.Address Then checkDuplicate = True

  If ActiveSheet.Index < ActiveWorkbook.Worksheets.Count Then
     For i = ActiveSheet.Index + 1 To ActiveWorkbook.Worksheets.count
        With Worksheets(i)
           Set rng = .Cells.Find(What:=TelNo, LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, MatchCase:=False)
           If Not rng Is Nothing Then checkDuplicate = True
        End With
     Next i
   End If
End Function

Notice I added a . before the range.find call. That is for the With Worksheets(i) to take effect. This is also the reason why I removed the Worksheets(i).Activate , since it is unnecessary when fully qualifying the sheets.
Another thing to note: Range(ActiveCell.Address).Name = "StartCell" is implicitly called on ActiveSheet .

You should avoid using ActiveSomething in your code in general, as it can cause a great deal of errors.