If a long paragraph is copied into the editor, <br> tags appear in thea content without being added at first place. 

For the time being you can resolve this by adding the following script below the editor declaration:

<telerik:RadEditor runat="server" ID="RadEditor1" StripFormattingOptions="AllExceptNewLines"></telerik:RadEditor>

<script type="text/javascript">
    var oldStripFormatting = Telerik.Web.UI.Editor.Utils.stripFormatting;

    Telerik.Web.UI.Editor.Utils.stripFormatting = function (textHtml, clearValue) {
        if (clearValue === "ALL_NO_BRAKES") {
            textHtml = textHtml.replace(/\n/g, "");
            textHtml = textHtml.replace(/<\/p>/g, "<br/>");
        }

        textHtml = oldStripFormatting.call(this, textHtml, clearValue);
        return textHtml;
    }
</script>