Cannot have multiple items selected in a DropDownList when edit the forum in sharepoint

hi ,

i  have custom forum for new and edit, in new forum we have controls drop down values, when select drop down1   values that related values are displayed in dropdown2 , when edit  item  that  time we getting error

Cannot have multiple items selected in a DropDownList. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: Cannot have multiple items selected in a DropDownList.

Source Error: 

HttpException (0x80004005): Cannot have multiple items selected in a DropDownList.]
   System.Web.UI.WebControls.DropDownList.VerifyMultiSelect() +124
   System.Web.UI.WebControls.ListControl.RenderContents(HtmlTextWriter writer) +10956501
   System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) +42
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.WebControls.WebControl.RenderContents(HtmlTextWriter writer) +13
   System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) +42
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +42
   ASP._controltemplates_amat_appayrequest_webparts_appayrequest_appayrequestusercontrol_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\CONTROLTEMPLATES\AMAT.APPayRequest.Webparts\APPayRequest\APPayRequestUserControl.ascx:350
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +115
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +42
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +253
   System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +87
   System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) +53
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +42
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +240
   System.Web.UI.Page.Render(HtmlTextWriter writer) +38
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4240