Rich text box control gets duplicated on partial post back - asp.net

Question

The rich text box (SharePoint control) gets duplicated on partial postbackl. How to overcome this issue? This control is outside the update panel.
<asp:UpdatePanel ID="ExternalCCUpdatePanel" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<table>
<tr>
<td width="45px"></td>
<td>
<asp:ListBox Width="182px" Height="200px" Visible="true" ID="ccEmailListBox" SelectionMode="Multiple" runat="server" ToolTip="Select external user/group names"></asp:ListBox>
</td>
<td width="22px"></td>
<td width="60px">
<asp:Button Width="50px" ID="addToCCSelectedBoxButton" CausesValidation="false" CssClass="submitButton" Text="Add >" runat="server" OnClick="addToCCSelectedBoxButton_Click" /><br />
<asp:Button Width="50px" ID="removeFromCCSelectedListBox" CausesValidation="false" CssClass="submitButton" Text="< Remove" runat="server" OnClick="removeFromCCSelectedListBox_Click" />
</td>
<td width="22px"></td>
<td>
<asp:ListBox Width="182px" Height="200px" Visible="true" SelectionMode="Multiple" ID="ccEmailSelectedListBox" runat="server"></asp:ListBox>
</td>
<td>
<asp:UpdateProgress ID="ExternalCCUpdateProgress" runat="server" AssociatedUpdatePanelID="ExternalCCUpdatePanel" DynamicLayout="true">
<ProgressTemplate>
<img src="/progress.gif"></img><br />
Please Wait...
</ProgressTemplate>
</asp:UpdateProgress>
</td>
</tr>
</table>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="addToCCSelectedBoxButton" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="removeFromCCSelectedListBox" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
<SharePoint:InputFormTextBox ID="RichTextBox"
RichText="true" Rows="20"
RichTextMode="FullHtml" runat="server"
TextMode="MultiLine">
</SharePoint:InputFormTextBox>
The problem occurs when the buttons are clicked inside the update panel. The functionality of the buttons are to add/remove entries from the list boxes.

Answer 1

Please try 2 different things. One - remove all the content from the update panel except the buttons and see if the problem persists. If it does I would consider looking at any master pages you have wrapped around this page and try and verify all your controls and html tags are property closed.
Second, try wrapping the richtextbox in a separate update panel and marking the mode conditional.
From the information you have I can't see what is causing the problem, but this may help fix the error or give you other places to look for the root cause.