If partial string = "dup" is in Column A, insert a row below that cell.

Hello Niedermee,

Here is another method. The assumptions are column "C" is empty, the row below the last row with data is empty, and the data is in ascending order as shown.

Sub InsertAfterDup()

    Dim DataRng As Range
    Dim LR      As Long
    Dim n       As Long
    Dim nMax    As Variant
    Dim Wks     As Worksheet
    
        Set Wks = ActiveSheet
        Set DataRng = Wks.Range("A1").CurrentRegion
        
        Application.ScreenUpdating = False
        
            Set DataRng = Intersect(DataRng, DataRng.Offset(1, 0))
            LR = DataRng.Rows.Count
            
            For n = 2 To LR + 1
                If DataRng.Item(n, 1) Like DataRng.Item(n - 1, 1) & "*" Then
                    nMax = Application.Max(DataRng.Item(n - 1, 2), DataRng.Item(n, 2))
                    DataRng.Item(n + 1, 1).EntireRow.Insert
                    DataRng.Item(n + 1, 1) = DataRng(n, 1) & "-max"
                    DataRng.Item(n + 1, 2) = nMax
                    LR = LR + 1
                    n = n + 1
                End If
            Next n
            
        Application.ScreenUpdating = True
        
End Sub