When EnableComments is set to true, the content from the clipboard is pasted twice in an empty editor.

RadEditor Configuration:

<telerik:RadEditor runat="server"
    EnableComments="true">
    <Tools>
        <telerik:EditorToolGroup>
            <telerik:EditorTool Name="Bold" />
            <telerik:EditorTool Name="Italic" />
            <telerik:EditorTool Name="Underline" />
            <telerik:EditorSeparator />
            <telerik:EditorTool Name="Undo" />
            <telerik:EditorTool Name="Redo" />
            <telerik:EditorTool Name="Cut" />
            <telerik:EditorTool Name="Copy" />
            <telerik:EditorTool Name="Paste" />
            <telerik:EditorTool Name="PastePlainText" Text="Paste Plain Text" />
            <telerik:EditorTool Name="FindAndReplace" Text="Find and Replace" />
            <telerik:EditorSeparator />
            <telerik:EditorTool Name="Indent" />
            <telerik:EditorTool Name="Outdent" />
            <telerik:EditorTool Name="InsertOrderedList" Text="Numbered List" />
            <telerik:EditorTool Name="InsertUnorderedList" Text="Bulleted List" />
            <telerik:EditorTool Name="LinkManager" />
            <telerik:EditorTool Name="Unlink" />
            <telerik:EditorSeparator />
            <telerik:EditorTool Name="AjaxSpellCheck" Text="Spell Check" />
            <telerik:EditorTool Name="InsertSymbol" Text="Insert Symbol" />
        </telerik:EditorToolGroup>
        <telerik:EditorToolGroup>
            <telerik:EditorTool Name="AcceptTrackChange" Text="Accept Change" />
            <telerik:EditorTool Name="RejectTrackChange" Text="Reject Change" />
            <telerik:EditorTool Name="AcceptAllTrackChanges" Text="Accept All Changes" />
            <telerik:EditorTool Name="RejectAllTrackChanges" Text="Reject All Changes" />
            <telerik:EditorTool Name="EnableTrackChangesOverride" Text="Track Changes" />
            <telerik:EditorTool Name="AddComment" Text="Add Comment" />
            <telerik:EditorTool Name="RemoveComment" Text="Remove Comment" />
            <telerik:EditorTool Name="RemoveAllComments" Text="Remove All Comments" />
            <telerik:EditorTool Name="ImageManager" Text="Insert Image" />
        </telerik:EditorToolGroup>
    </Tools>
</telerik:RadEditor>

Potential workaround:
<script type="text/javascript">
    function OnClientPasteHtml(sender, args) {
        var commandName = args.get_commandName();
        var value = args.get_value();
 
        if (commandName == "Paste" && Telerik.Web.Browser.ie) {
            args.set_value(value);
            args.set_cancel(true);
        }
    }
</script>
<telerik:RadEditor runat="server" Height="300px" StripFormattingOptions="Span,Font,Css" EnableTrackChanges="true" OnClientPasteHtml="OnClientPasteHtml"
    ContentFilters="FixEnclosingP,RemoveScripts,FixUlBoldItalic,IECleanAnchors,MozEmStrong,ConvertFontToSpan,ConvertToXhtml,IndentHTMLContent,EncodeScripts,OptimizeSpans,ConvertTags,StripCssExpressions,RemoveExtraBreaks"
    EnableComments="true" NewLineMode="P" EditModes="Design,Html">
</telerik:RadEditor>