diff --git a/Sources/ManagedModels/SchemaGeneration/SchemaBuilder.swift b/Sources/ManagedModels/SchemaGeneration/SchemaBuilder.swift
index ccc1b46..c5e0ff5 100644
--- a/Sources/ManagedModels/SchemaGeneration/SchemaBuilder.swift
+++ b/Sources/ManagedModels/SchemaGeneration/SchemaBuilder.swift
@@ -130,18 +130,20 @@ public final class SchemaBuilder {
         }
         assertionFailure("Type frozen, but no entity found?")
       }
+      
       allFrozen = false
       if let newEntity = processModel(modelType) {
         entities.append(newEntity)
       }
     }
-    if allFrozen { return } // all have been processed already
     
     // TBD: The following does too much work, we might only need the
     //      most of those on the "new models"
     
     // This recurses into `process`, if necessary.
     discoverTargetTypes(in: entities, allEntities: &entities)
+    
+    if allFrozen { return }
 
     // Collect destination entity names in relships based on the modelType!
     fillDestinationEntityNamesInRelationships(entities)
@@ -168,7 +170,16 @@ public final class SchemaBuilder {
           continue
         }
         // This returns nil if the model is already processed.
-        guard let newEntity = processModel(targetType) else { continue }
+        guard let newEntity = processModel(targetType) else { 
+          guard let existingEntity = lookupEntity(targetType) else {
+            assertionFailure("Type marked as processed, but no entity?")
+            continue
+          }
+          if !allEntities.contains(where: { $0 === existingEntity }) {
+            allEntities.append(existingEntity)
+          }
+          continue
+        }
         
         allEntities.append(newEntity)
         newEntities.append(newEntity)
diff --git a/Tests/ManagedModelTests/SchemaGenerationTests.swift b/Tests/ManagedModelTests/SchemaGenerationTests.swift
index bdcb725..4e51543 100644
--- a/Tests/ManagedModelTests/SchemaGenerationTests.swift
+++ b/Tests/ManagedModelTests/SchemaGenerationTests.swift
@@ -193,4 +193,33 @@ final class SchemaGenerationTests: XCTestCase {
     XCTAssertNotNil(mom.entitiesByName["Person"])
     XCTAssertNotNil(mom.entitiesByName["Address"])
   }
+  
+  func testDuplicateGeneration() throws {
+    let cache = SchemaBuilder()
+    
+    try autoreleasepool {
+      let entities = cache.lookupAllEntities(for: [
+        Fixtures.PersonAddressSchema.Person.self
+      ])
+      XCTAssertEqual(entities.count, 2)
+      
+      let address = try XCTUnwrap(
+        entities.first(where: { $0.name == "Address" })
+      )
+      XCTAssertEqual(address.attributes.count, 2)
+    }
+
+    // second run
+    try autoreleasepool {
+      let entities = cache.lookupAllEntities(for: [
+        Fixtures.PersonAddressSchema.Person.self
+      ])
+      XCTAssertEqual(entities.count, 2)
+      
+      let address = try XCTUnwrap(
+        entities.first(where: { $0.name == "Address" })
+      )
+      XCTAssertEqual(address.attributes.count, 2)
+    }
+  }
 }