When implementing interfaces that "inherits" other interfaces that contains indexes, it looks like the index get's registered double. This is the stack trace I get. Basically IEmployee inherits IPerson and IPerson has a property called Country on it.

[ArgumentException: An item with the same key has already been added.] System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +52 System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add) +9382923 Xtensive.Orm.Model.NodeCollection1.OnInserted(TNode value, Int32 index) +45 Xtensive.Collections.CollectionBase1.Add(TItem value) +134 Xtensive.Orm.Model.NodeCollection1.Add(TNode item) +45

[InvalidOperationException: Item with name 'IEmployee.IPerson.FK_Country.UNION' already exists in 'IEmployee.Indexes'.] Xtensive.Orm.Model.NodeCollection1.Add(TNode item) +131 Xtensive.Orm.Building.Builders.IndexBuilder.CreateInterfaceIndexes(TypeInfo interface, ICollection1 processedInterfaces) +553 Xtensive.Orm.Building.Builders.IndexBuilder.CreateInterfaceIndexes() +131 Xtensive.Orm.Building.Builders.IndexBuilder.BuildIndexes() +100 Xtensive.Orm.Building.Builders.ModelBuilder.BuildModel() +146 Xtensive.Orm.Building.Builders.ModelBuilder.Run() +268 Xtensive.Orm.Building.Builders.DomainBuilder.BuildModel() +119 Xtensive.Orm.Building.Builders.DomainBuilder.BuildDomain(DomainConfiguration configuration, DomainBuilderConfiguration builderConfiguration) +258 Xtensive.Orm.Upgrade.UpgradingDomainBuilder.BuildStageDomain(UpgradeStage stage) +296 Xtensive.Orm.Upgrade.UpgradingDomainBuilder.Build(DomainConfiguration configuration) +232 Xtensive.Orm.Domain.Build(DomainConfiguration configuration) +5