Handle exception inside Application_Error global.asax - asp.net

Question

I have code written in global file on Application_Error.
I am inserting the error detail in my database.
But some how sometimes it is inserting thousand record of same error.
Does this mean that there is an error inside Application_Error
If yes then how can I handle that.
Edit 1
I am also redirecting to error page after inserting the error.
Error detail
Message: Exception of type 'System.Web.HttpUnhandledException' was thrown.<br><br><br>
Inner Exception : System.NullReferenceException: Object reference not set to an instance of an object.
at System.Collections.Specialized.NameObjectCollectionBase.BaseGetKey(Int32 index)
at System.Collections.Specialized.NameObjectCollectionBase.BaseRemove(String name)
at System.Web.SessionState.SessionStateItemCollection.Remove(String name)
at InfoDomeWeb.UserControl.LeftPanel.SetCustomerInfo() in D:\Working Folder\InfoDome\InfoDomeWeb\UserControl\LeftPanel.ascx.cs:line 185
at InfoDomeWeb.UserControl.LeftPanel.Page_Load(Object sender, EventArgs e) in D:\Working Folder\InfoDome\InfoDomeWeb\UserControl\LeftPanel.ascx.cs:line 71
at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)<br><br><br>Stack trace: at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.error_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\961a93aa\98c42fe5\App_Web_304di5zu.12.cs:line 0
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
The error occurred on the page
D:\Working Folder\InfoDome\InfoDomeWeb\UserControl\LeftPanel.ascx.cs
But I don't find any thing wrong there.