We have the same problem, in multiple projects. As soon as we copy the EPiServer.Forms_SV.xml(the one included in the nuget package) to our resource directory (where we keep all our language files) we get this error. 

Removing the section episerver -> forms -> formscomponent in EPiServer.Forms_SV.xml makes the "/ClientResources/epi-cms/nls/sv-se/episerver.forms.formscomponent.js" url work.
The same thing applies to the "/ClientResources/epi-cms/nls/sv-se/episerver.forms.formscontentarea.js" url.

Stacktrace:

[ArgumentException: An item with the same key has already been added.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +56
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +13737545
   EPiServer.Shell.UI.Controllers.EPiResourcesController.GetResourceDictionary(String resourceKey, CultureInfo culture) +674
   EPiServer.Shell.UI.Controllers.EPiResourcesController.DojoResources(String pathInfo) +589
   lambda_method(Closure , ControllerBase , Object[] ) +139
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +229
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +35
   System.Web.Mvc.<>c__DisplayClass15.<InvokeActionMethodWithFilters>b__12() +80
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +453
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +453
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +453
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +453
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +982
   System.Web.Mvc.Controller.ExecuteCore() +129
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +236
   EPiServer.Shell.Web.Mvc.ModuleMvcHandler.ProcessController(IController controller) +50
   EPiServer.Shell.Web.Mvc.ModuleMvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +28
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +923
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +137