Unable to open project (#226)

Get the following when opening some projects created with earlier versions:

System.ArgumentException: An item with the same key has already been added.

at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)

at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)

at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)

at System.Linq.Enumerable.ToDictionary(IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)

at System.Linq.Enumerable.ToDictionary(IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector)

at SoftwareIdeasModeler.Core.DiagramItems.UmlObjects.UmlObject.BindToEntities(Project project)

at SoftwareIdeasModeler.Core.Projecting.Project.BindAbstractItem(ModelElement item)

at SoftwareIdeasModeler.Core.Projecting.Project.BindEntities()

at SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

at SoftwareIdeasModeler.ProjectHelper.OpenProject(MainForm mainForm, String filename)