Exception has been thrown

PLEASE HELP. It keeps saying this and i dont know what is wrong. please help asap.

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[General Info]

Application: SambaPOS
Version:     4.1.82
Region:      en
DB:          SQ
Machine:     NORTHFORK
User:        Todd
Date:        3/27/2015
Time:        9:39 PM

User Explanation:

Todd said ""
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[Exception Info 1]

Top-level Exception
Type:        System.Reflection.TargetInvocationException
Message:     Exception has been thrown by the target of an invocation.
Source:      mscorlib
Stack Trace: at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.Run()
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at Samba.Presentation.App.Main()

Inner Exception 1
Type:        System.Reflection.TargetInvocationException
Message:     Exception has been thrown by the target of an invocation.
Source:      mscorlib
Stack Trace: at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Threading.DispatcherOperation.Wait(TimeSpan timeout)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherOperation operation, CancellationToken cancellationToken, TimeSpan timeout)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at Samba.Presentation.Services.Common.ExtensionMethods.PublishEvent[TEventsubject](TEventsubject eventArgs, String eventTopic, Boolean wait)
   at Samba.Presentation.Shell.<.ctor>b__1(EventParameters`1 x)
   at Microsoft.Practices.Prism.Events.EventSubscription`1.InvokeAction(Action`1 action, TPayload argument)
   at Microsoft.Practices.Prism.Events.EventSubscription`1.<>c__DisplayClass2.<GetExecutionStrategy>b__0(Object[] arguments)
   at Microsoft.Practices.Prism.Events.EventBase.InternalPublish(Object[] arguments)
   at Microsoft.Practices.Prism.Events.CompositePresentationEvent`1.Publish(TPayload payload)
   at Samba.Presentation.Services.Common.ExtensionMethods.Publish[TEventsubject](TEventsubject eventArgs, String eventTopic, Action expectedAction)

Inner Exception 2
Type:        System.InvalidOperationException
Message:     Sequence contains more than one matching element
Source:      System.Core
Stack Trace: at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
   at Samba.Modules.PosModule.MenuItemSelectorViewModel.CreateCategoryButtons(ScreenMenu screenMenu)
   at Samba.Modules.PosModule.MenuItemSelectorViewModel.UpdateCurrentScreenMenu(Int32 screenMenuId)
   at Samba.Modules.PosModule.PosViewModel.<.ctor>b__0(EventParameters`1 x)
   at Microsoft.Practices.Prism.Events.EventSubscription`1.InvokeAction(Action`1 action, TPayload argument)
   at Microsoft.Practices.Prism.Events.EventSubscription`1.<>c__DisplayClass2.<GetExecutionStrategy>b__0(Object[] arguments)
   at Microsoft.Practices.Prism.Events.EventBase.InternalPublish(Object[] arguments)
   at Microsoft.Practices.Prism.Events.CompositePresentationEvent`1.Publish(TPayload payload)
   at Samba.Presentation.Services.Common.ExtensionMethods.Publish[TEventsubject](TEventsubject eventArgs, String eventTopic, Action expectedAction)

-----------------------------

[Assembly Info]

Samba.Services, Version=1.0.0.0
mscorlib, Version=4.0.0.0
System, Version=4.0.0.0
Microsoft.Practices.Prism, Version=4.0.0.0
WindowsBase, Version=4.0.0.0
Samba.Infrastructure, Version=1.0.0.0
Samba.Infrastructure.Data, Version=1.0.0.0
PresentationFramework, Version=4.0.0.0
System.Xaml, Version=4.0.0.0
Samba.Presentation.Services, Version=1.0.0.0
FluentValidation, Version=3.4.0.0
Samba.Domain, Version=1.0.0.0
DevExpress.Xpf.Grid.v14.1, Version=14.1.6.0
DevExpress.Xpf.Grid.v14.1.Core, Version=14.1.6.0
PresentationCore, Version=4.0.0.0
Stateless, Version=1.0.0.0
System.Core, Version=4.0.0.0
System.Drawing, Version=4.0.0.0
DevExpress.Xpf.Core.v14.1, Version=14.1.6.0
EntityFramework, Version=4.4.0.0
System.Windows.Forms, Version=4.0.0.0
System.ComponentModel.Composition, Version=4.0.0.0
Samba.Localization, Version=1.0.0.0
Microsoft.CSharp, Version=4.0.0.0
Microsoft.Practices.ServiceLocation, Version=1.0.0.0
Samba.Persistance, Version=1.0.0.0
Microsoft.Practices.Prism.MefExtensions, Version=4.0.0.0
PropertyTools, Version=2012.4.14.1

-----------------------------

[System Info]

Operating System
-Microsoft Windows 8
--CodeSet = 1252
--CSDVersion = 
--CurrentTimeZone = -240
--FreePhysicalMemory = 2397540
--OSArchitecture = 64-bit
--OSLanguage = 1033
--ServicePackMajorVersion = 0
--ServicePackMinorVersion = 0
--Version = 6.2.9200

Machine
-NORTHFORK
--Manufacturer = Gateway
--Model = SX2110G
--TotalPhysicalMemory = 3995144192
--UserName = northfork\NORTHFORK\Todd

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In your menu did you try to make two categories named the same? If you could share what you were doing it might help us.

These two are key. If you explained more of what you did before it happened we could help more.