新增窗体重复值不能判断

在平台开发中，新增窗体总是不能判断重复值，不知道为什么，代码应该没有问题呀！代码如下：

Dim rst As DAO.Recordset

If IsNull(Me.TeacherName) Then
          MsgBox "请输入教师姓名!", vbCritical, "提示："
      Me.TeacherName.SetFocus
          Exit Sub
End If
Me.Refresh
If Acchelp_StrDataIsExist("TblTeacher", "TeacherName", Me.TeacherName) = True Then
          MsgBox "你输入的教师已经存在，请重新输入！", vbCritical, "警告"
     Me.TeacherName.SetFocus
          Exit Sub
End If
If MsgBox("您确认要保存吗?", vbOKCancel + vbInformation, "提示") = vbOK Then
         Set rst = CurrentDb.OpenRecordset("Tblteacher", dbOpenDynaset)
         rst.AddNew
         rst("TeacherId") = acchelp_autoid("JS", 3, "tblTeacher", "TeacherId")
         rst("TeacherName") = Me.TeacherName
         rst.Update
         rst.Close
        Set rst = Nothing
        '刷新数据
        If IsLoaded("usysfrmMain") Then
               DoCmd.Echo False
               Forms!usysfrmMain!frmChild.SourceObject = "frmTeacher_child"
               DoCmd.Echo True
        End If
        MsgBox "保存成功!", vbInformation, "提示"
        Me.TeacherName = Null

End If

文本框名称为TeacherName 表名为TblTeacher 表中字段为TeacherId，TeacherName

请老师帮忙看看