Editing post sequence error

Server Error in '/' Application.

Sequence contains more than one element

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.InvalidOperationException: Sequence contains more than one element

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: Sequence contains more than one element]
   System.Data.Linq.SqlClient.SqlProvider.Execute(Expression query, QueryInfo queryInfo, IObjectReaderFactory factory, Object[] parentArgs, Object[] userArgs, ICompiledSubQuery[] subQueries, Object lastResult) +1191
   System.Data.Linq.SqlClient.SqlProvider.ExecuteAll(Expression query, QueryInfo[] queryInfos, IObjectReaderFactory factory, Object[] userArguments, ICompiledSubQuery[] subQueries) +118
   System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query) +342
   System.Data.Linq.Table`1.System.Linq.IQueryProvider.Execute(Expression expression) +58
   System.Linq.Queryable.SingleOrDefault(IQueryable`1 source, Expression`1 predicate) +287
   v2.ArticleFunctions.GetImageSize(String url) +415
   ArticlesCommon.Markdown(String Input) +1299
   TutorialSubmit.SubmitTutorial(Object sender, EventArgs e) +4067
   System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) +116
   System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) +101
   System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +9642338
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1724

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18044