An entire site is down with this error. DNN 6.x - don't have the specific version number at hand.

Sequence contains more than one matching 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 matching element

Source Error: 

Stack Trace: 

[InvalidOperationException: Sequence contains more than one matching element]
   System.Linq.Enumerable.SingleOrDefault(IEnumerable`1 source, Func`2 predicate) +2589154
   DotNetNuke.HttpModules.Membership.MembershipModule.AuthenticateRequest(HttpContextBase context, Boolean allowUnknownExtensinons) +609
   DotNetNuke.HttpModules.Membership.MembershipModule.OnAuthenticateRequest(Object sender, EventArgs e) +77
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

I read another thread with a similar error and it suggested an IIS app pool refresh might help (unable to try that at the moment) and that there's probably a duplicate entry in the database. Suggestions on the best way to troubleshoot and fix this error? Thanks!

Thanks - that got the site running again. DNN version is 6.02.00. Any thoughts on what may have generated that error? Haven't had that happen before and would like to prevent it in the future if possible.