This is simple enough in Excel VBA. Access the VBE by pressing Alt-F11 and insert a new module. Copy and paste the following code into the module and run it. Back up your file first before doing this.
Sub RemoveRepeatingStrings()
Dim BaseStr As String, CurrStr As String
Dim EndRow As Long
EndRow = Range("A" & Rows.Count).End(xlUp).Row
BaseStr = Range("A1").Value
Application.ScreenUpdating = False
For Iter = 2 To EndRow
CurrStr = Range("A" & Iter).Value
If CurrStr = BaseStr Then
Range("A" & Iter).Value = vbNullString
Else
BaseStr = Range("A" & Iter).Value
End If
Next Iter
Application.ScreenUpdating = True
End Sub
Set-up:
Result:
Let us know if this helps.