Possible exception when validating inventory with workflow

Description

Reported from http://world.episerver.com/forum/developer-forum/Episerver-Commerce/Thread-Container/2017/9/workflow-exception-after-upgrade-to-commerce-11/?pageIndex=1#reply

If a cart has two or more lineitems which do not have enough quantity in inventory, an exception might be thrown.

2017-09-17T23:43:55,Error,scdomstr012im9nprod,943134,636412886352348485,0,3024,70,"EPiServer.Global : 1.2.5 Unhandled exception in ASP.NET
System.ApplicationException: Workflow generated an exception, please look at the previous error for more details. ---> System.ArgumentException: Item has already been added. Key in dictionary: 'rejectedinventoryduetoinsufficientquantity' Key being added: 'rejectedinventoryduetoinsufficientquantity'
at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
at Mediachase.Commerce.Workflow.Activities.Cart.AdjustInventoryActivity.RequestInventory(OrderForm orderForm, Shipment shipment, LineItem lineItem)
at Mediachase.Commerce.Workflow.Activities.Cart.AdjustInventoryActivity.Execute(ActivityExecutionContext executionContext)
at Mediachase.Commerce.WorkflowCompatibility.Activity.Execute()
at Mediachase.Commerce.Engine.ActivityFlowRunner.<>c_DisplayClass15_0`1.<Do>b_0()
at Mediachase.Commerce.Engine.ActivityFlowRunner.Execute()
at Mediachase.Commerce.Engine.ExecutionManager.ExecuteActivityFlow(String name, ActivityFlowContext context)