先判断，再决定是否新增

If e.DataCol.Name = "逻辑列名" AndAlso e.DataRow("逻辑列名") = True Then
    Dim dr As DataRow = DataTables("表B").Find("某列 = \'" & e.DataRow("某列") & "\'")
    If dr Is Nothing Then dr = DataTables("表B").AddNew
    For Each dc As DataCol In DataTables("表B").DataCols
        dr(dc.Name) = e.DataRow(dc.Name)
    Next
End If

比如，检测历史记录表DataColChanged事件，写代码

If e.DataCol.Name = "自动生成" AndAlso e.DataRow("自动生成") = True Then
    Dim dr As DataRow = DataTables("隐患登记表").Find("车号 = \'" & e.DataRow("车号") & "\' and 日期 = #" & e.DataRow("检查日期") & "#")
    If dr Is Nothing Then dr = DataTables("隐患登记表").AddNew
    Dim nma() As String = {"车号","受控名称","检查日期"} \'A表数据来源列
    Dim nmb() As String = {"车号","隐患部位","日期"} \'B表数据接收列
    For i As Integer = 0 To nma.Length - 1
        dr(nmb(i)) = e.DataRow(nma(i))
    Next
End If

Select Case e.DataCol.name
    Case "车号","点位","描述","点检组归档"
        Dim dr As DataRow = e.DataRow     \'将上述车号/点位/描述/点检组归档的值赋给dr变量.
        If e.DataRow("点检组归档")=Nothing Then  \'如果点检组归档为空时
            If dr.IsNull("车号") = False AndAlso dr.IsNull("点位") = False AndAlso dr.IsNull("描述") = False AndAlso dr.IsNull("点检组归档")= False Then \'如果左侧列都不为空.
                If e.DataTable.Compute("Count([_Identify])","车号 = \'" & dr("车号") & "\'And 点位 = \'" & dr("点位") & "\' And 描述 = \'" & dr("描述") & "\' And 点检组归档= \'" & dr("点检组归档") & "\'") > 1 Then
                   
                    MessageBox.Show("相同记录已经存在!不准重复录入","提示",MessageBoxButtons.OK,MessageBoxIcon.Information)

                    DataTables("检测历史记录").deletefor("车号 = \'" & dr("车号") & "\'And 点位 = \'" & dr("点位") & "\' And 描述 = \'" & dr("描述") & "\' And 点检组归档= \'" & dr("点检组归档") & "\'")
                    e.DataRow.Delete  \'删除当前准备录入的行                   
                End If
            End If
        End If
End Select