Trigger that will update cells in column of the same value - excel

Question

I am trying to write a macro that will update all cells in a column that have the same value as the adjacent column below are before and after of what I am trying to accomplish. In this example you would update B1 and then any cells in A1 with the same value would update to the B1 value
Here is the code I am using
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng1 As Range
Dim cel As Range
Set rng1 = Range("A1", Range("A2").End(xlDown))
For Each cel In rng1
If cel = Target.Offset(0, -1).Value Then
cel.Offset(0, 1).Value = Target.Value
End If
Next cel
End Sub
I am not sure if what I wrote is correct, but I keep getting out of stack space error, which I think is from the macro continuously looping every time through changing the same cells. I believe this should be possible but I am a little lost.

Answer 1

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column <> 2 Then Exit Sub
Application.ScreenUpdating = False
For Each cel In Range("A1:A" & Range("A" & Rows.Count).End(xlUp).Row)
If cel = Target.Offset(, -1) Then
cel.Offset(, 1) = Target
End If
Next cel
Application.ScreenUpdating = True
End Sub

Answer 2

I would try to avoid looping if possible. Perhaps use a UDF instead, using the .Find() method?
Option Explicit
Function myLookup(ByVal rng As Range) As String
Application.Volatile
Dim ws As Worksheet, lookupRng As Range, retRng As Range
Set ws = rng.Parent
With ws
Set lookupRng = .Range(.Cells(1, rng.Column), .Cells(rng.Row - 1, rng.Column))
End With
Set retRng = lookupRng.Find(rng.Value, ws.Cells(1, rng.Column))
If retRng Is Nothing Then
myLookup = vbNullString
Else
With retRng
myLookup = ws.Cells(.Row, .Column + 1)
End With
End If
End Function
You would place this UDF in the worksheet as follows:
and fill down. This will prevent circular references because it will search for the cells above it only within the lookupRng.
And, the final result: