from what i can gather the mailbox bit of the web interface works fine.. when any of the users go to options (top right) and try to use some of the features such as the
Organise Mail > Delivery Reports to find messages etc... it comes up with a message ..
"An item with the same key has already been added"

I've looked in the event viewer and i think its this error ->

Watson report about to be sent for process id: 7016, with parameters: E12IIS, c-RTL-AMD64, 14.00.0639.021, ECP, ECP.Powershell, https://x.x.x.x/ecp/PersonalSettings/Accounts.svc/GetList, UnexpectedCondition:ArgumentException, c09, 14.00.0639.021. ErrorReportingEnabled: False

and

Request for URL 'https://x.x.x.x/ecp/PersonalSettings/Accounts.svc/GetList' failed with the following error: System.ArgumentException: An item with the same key has already been added. at System.ServiceModel.AsyncResult.End[TAsyncResult](IAsyncResult result) at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.End(IAsyncResult result) at System.ServiceModel.Activation.HostedHttpRequestAsyncResult.ExecuteSynchronous(HttpApplication context, Boolean flowContext) at Microsoft.Exchange.Management.ControlPanel.WebServiceHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

I've tried googling but no luck that's relevant :(