diff --git a/Orm/Xtensive.Orm.Tests/Storage/ClassTableMultiPathInterfaceTest.cs b/Orm/Xtensive.Orm.Tests/Storage/ClassTableMultiPathInterfaceTest.cs
new file mode 100644
index 000000000..45dbb7981
--- /dev/null
+++ b/Orm/Xtensive.Orm.Tests/Storage/ClassTableMultiPathInterfaceTest.cs
@@ -0,0 +1,106 @@
+// Copyright (C) 2025 Xtensive LLC.
+// This code is distributed under MIT license terms.
+// See the License.txt file in the project root for more information.
+
+using NUnit.Framework;
+using Xtensive.Orm.Model;
+using Xtensive.Orm.Tests.Storage.ClassTableMultiPathInterfaceTestModel;
+
+namespace Xtensive.Orm.Tests.Storage
+{
+  namespace ClassTableMultiPathInterfaceTestModel
+  {
+    [HierarchyRoot]
+    public class Owner : Entity
+    {
+      [Field, Key]
+      public int Id { get; private set; }
+    }
+
+    // Persistent interface that declares an Entity-reference field. Xtensive auto-emits an
+    // FK_Owner index for it.
+    public interface IOwned : IEntity
+    {
+      [Field]
+      Owner Owner { get; }
+    }
+
+    // Sub-interface of IOwned. Its Indexes collection independently carries an inherited
+    // copy of the FK_Owner IndexInfo (a different IndexInfo reference whose DeclaringIndex
+    // chain bottoms out at IOwned.FK_Owner).
+    public interface INamedAndOwned : IOwned
+    {
+      [Field(Length = 50)]
+      string Label { get; set; }
+    }
+
+    public abstract class OwnedBase : Entity, IOwned
+    {
+      [Field, Key]
+      public int Id { get; private set; }
+
+      [Field]
+      public Owner Owner { get; set; }
+    }
+
+    public abstract class NamedOwnedBase : OwnedBase, INamedAndOwned
+    {
+      public string Label { get; set; }
+    }
+
+    // typeof(Root).GetInterfaces() returns both IOwned (directly via OwnedBase) and
+    // INamedAndOwned (via NamedOwnedBase). Xtensive's ModelInspector.FindInterfaces uses
+    // .NET reflection's full transitive set, so DirectInterfaces on Root contains both.
+    [HierarchyRoot(InheritanceSchema.ClassTable)]
+    public class Root : NamedOwnedBase
+    {
+      [Field]
+      public int Payload { get; set; }
+    }
+
+    // HierarchyInfo.UpdateState() demotes single-type hierarchies to ConcreteTable, which
+    // would mask the ClassTable assertion below. A concrete descendant keeps Types.Count > 1.
+    public class Leaf : Root
+    {
+      [Field]
+      public int Extra { get; set; }
+    }
+  }
+
+  /// <summary>
+  /// Regression test for the ClassTable inherited-interface dedup bug.
+  ///
+  /// Before the fix, BuildClassTableIndexes used reference identity on the visited
+  /// interfaceIndex:
+  ///
+  ///   if (type.Indexes.Any(i => i.DeclaringIndex == interfaceIndex)) continue;
+  ///
+  /// When the same logical interface index reached a ClassTable root through two paths
+  /// in DirectInterfaces, each path yielded a distinct IndexInfo. The guard missed on
+  /// the second visit; BuildInheritedIndex ran again and produced an index with the
+  /// same name; NodeCollection.Add threw:
+  ///
+  ///   "Item with name 'Root.IOwned.FK_Owner' already exists in 'Root.Indexes'"
+  ///
+  /// at Domain.Build, before any DB work.
+  ///
+  /// Sibling BuildConcreteTableIndexes already handles this with a name-based check
+  /// after BuildInheritedIndex (disposing the surplus IndexInfo); BuildSingleTableIndexes
+  /// handles it with a chain-normalized identity check. ClassTable was the odd one out.
+  /// </summary>
+  [TestFixture]
+  public class ClassTableMultiPathInterfaceTest
+  {
+    [Test]
+    public void Build_ClassTableHierarchy_WithMultiPathInterfaceImpl_Succeeds()
+    {
+      var configuration = DomainConfigurationFactory.Create();
+      configuration.Types.Register(typeof(Owner).Assembly, typeof(Owner).Namespace);
+      using (var domain = Domain.Build(configuration)) {
+        Assert.That(domain.Model.Types[typeof(Root)], Is.Not.Null);
+        Assert.That(domain.Model.Types[typeof(Root)].Hierarchy.InheritanceSchema,
+          Is.EqualTo(InheritanceSchema.ClassTable));
+      }
+    }
+  }
+}
diff --git a/Orm/Xtensive.Orm/Orm/Building/Builders/IndexBuilder.ClassTable.cs b/Orm/Xtensive.Orm/Orm/Building/Builders/IndexBuilder.ClassTable.cs
index e9dccce41..4c36f8c88 100644
--- a/Orm/Xtensive.Orm/Orm/Building/Builders/IndexBuilder.ClassTable.cs
+++ b/Orm/Xtensive.Orm/Orm/Building/Builders/IndexBuilder.ClassTable.cs
@@ -68,11 +68,20 @@ private void BuildClassTableIndexes(TypeInfo type)
           var index = BuildInheritedIndex(type, interfaceIndex, false);
           if (IndexBuiltOverInheritedFields(index)) {
             BuildLog.Warning(string.Format(Strings.ExUnableToBuildIndexXBecauseItWasBuiltOverInheritedFields, index.Name));
+            index.Dispose();
+            continue;
           }
-          else {
-            type.Indexes.Add(index);
-            context.Model.RealIndexes.Add(index);
+          // The same logical interface index can arrive through multiple paths in DirectInterfaces
+          // (e.g. a type implements IBusinessEntity directly via one base AND transitively via a
+          // sub-interface like INamedBusinessEntity). The identity-equality guard above misses this
+          // because each path yields a distinct IndexInfo. Mirror BuildConcreteTableIndexes' name-
+          // based guard after BuildInheritedIndex to drop the redundant emission.
+          if (type.Indexes.Contains(index.Name)) {
+            index.Dispose();
+            continue;
           }
+          type.Indexes.Add(index);
+          context.Model.RealIndexes.Add(index);
         }
       }