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Sub UnmergeFill()

Dim cell As Range, joinedCells As Range
Application.ScreenUpdating = False

For Each cell In ActiveWorkbook.ActiveSheet.UsedRange
    If cell.MergeCells Then
        Set joinedCells = cell.MergeArea
        cell.MergeCells = False
        joinedCells.Value = cell.Value
    End If
Next

Application.ScreenUpdating = True

End Sub