diff --git a/packages/composer-common/lib/modelmanager.js b/packages/composer-common/lib/modelmanager.js
index 62604f1020..cd40f39949 100644
--- a/packages/composer-common/lib/modelmanager.js
+++ b/packages/composer-common/lib/modelmanager.js
@@ -134,8 +134,13 @@ class ModelManager {
             throw new Error('Cannot add a model file with the reserved system namspace: ' + m.getNamespace() );
         }
 
-        m.validate();
-        this.modelFiles[m.getNamespace()] = m;
+        if (!this.modelFiles[m.getNamespace()]) {
+            m.validate();
+            this.modelFiles[m.getNamespace()] = m;
+        } else {
+            throw new Error('namespace already exists');
+        }
+
         return m;
     }
 
@@ -221,14 +226,26 @@ class ModelManager {
                     if (m.isSystemModelFile()){
                         throw new Error('System namespace can not be updated');
                     }
-                    this.modelFiles[m.getNamespace()] = m;
-                    newModelFiles.push(m);
+
+                    if (!this.modelFiles[m.getNamespace()]) {
+                        this.modelFiles[m.getNamespace()] = m;
+                        newModelFiles.push(m);
+                    }
+                    else {
+                        throw new Error('namespace already exists');
+                    }
                 } else {
                     if (modelFile.isSystemModelFile()){
                         throw new Error('System namespace can not be updated');
                     }
-                    this.modelFiles[modelFile.getNamespace()] = modelFile;
-                    newModelFiles.push(modelFile);
+
+                    if (!this.modelFiles[modelFile.getNamespace()]) {
+                        this.modelFiles[modelFile.getNamespace()] = modelFile;
+                        newModelFiles.push(modelFile);
+                    }
+                    else {
+                        throw new Error('namespace already exists');
+                    }
                 }
             }
 
diff --git a/packages/composer-common/test/codegen/loopbackvisitor.js b/packages/composer-common/test/codegen/loopbackvisitor.js
index 25730fecb0..f31c696a9c 100644
--- a/packages/composer-common/test/codegen/loopbackvisitor.js
+++ b/packages/composer-common/test/codegen/loopbackvisitor.js
@@ -918,7 +918,7 @@ describe('LoopbackVisitor', () => {
                 });
 
                 it('should use the model file of the referencing type to resolve enumeration types', () => {
-                    modelManager.addModelFile(`
+                    modelManager.updateModelFile(`
                     namespace org.acme.base
                     enum Enum {
                         o SOME_VALUE
@@ -1021,7 +1021,7 @@ describe('LoopbackVisitor', () => {
                 });
 
                 it('should use the model file of the referencing type to resolve other types', () => {
-                    modelManager.addModelFile(`
+                    modelManager.updateModelFile(`
                     namespace org.acme.base
                     asset MyInlineAsset identified by assetId {
                         o String assetId
diff --git a/packages/composer-common/test/introspect/classdeclaration.js b/packages/composer-common/test/introspect/classdeclaration.js
index 8a83f3e808..a0637c9511 100644
--- a/packages/composer-common/test/introspect/classdeclaration.js
+++ b/packages/composer-common/test/introspect/classdeclaration.js
@@ -55,7 +55,7 @@ describe('ClassDeclaration', () => {
             const modelFile = new ModelFile(modelManager, modelDefinitions);
             modelFiles.push(modelFile);
         }
-        modelManager.addModelFiles(modelFiles, modelFileNames);
+        //modelManager.addModelFiles(modelFiles, modelFileNames);
         return modelFiles;
     };
 
diff --git a/packages/composer-common/test/modelmanager.js b/packages/composer-common/test/modelmanager.js
index 88ce2e5e9c..49c5a6b4f7 100644
--- a/packages/composer-common/test/modelmanager.js
+++ b/packages/composer-common/test/modelmanager.js
@@ -126,6 +126,24 @@ describe('ModelManager', () => {
             }).should.throw();
         });
 
+        it('should return error for duplicate namespaces for a string', () => {
+            modelManager.addModelFile(modelBase);
+            let mf1 = sinon.createStubInstance(ModelFile);
+            mf1.getNamespace.returns('org.acme.base');
+            (() => {
+                modelManager.addModelFile(modelBase);
+            }).should.throw(/namespace already exists/);
+        });
+
+        it('should return error for duplicate namespaces from an object', () => {
+            modelManager.addModelFile(modelBase);
+            let mf1 = sinon.createStubInstance(ModelFile);
+            mf1.getNamespace.returns('org.acme.base');
+            (() => {
+                modelManager.addModelFile(mf1);
+            }).should.throw(/namespace already exists/);
+        });
+
     });
 
     describe('#addModelFiles', () => {
@@ -157,7 +175,7 @@ describe('ModelManager', () => {
         it('should add to existing model files from strings', () => {
             modelManager.addModelFile(modelBase);
             modelManager.getModelFile('org.acme.base').getNamespace().should.equal('org.acme.base');
-            modelManager.addModelFiles([modelBase, concertoModel, farm2fork]);
+            modelManager.addModelFiles([concertoModel, farm2fork]);
             modelManager.getModelFile('org.acme.base').getNamespace().should.equal('org.acme.base');
             modelManager.getModelFile('org.acme').getNamespace().should.equal('org.acme');
         });
@@ -215,6 +233,24 @@ describe('ModelManager', () => {
                 modelManager.addModelFiles([mockSystemModelFile]);
             }).should.throw();
         });
+
+        it('should return an error for duplicate namespace from strings', () => {
+            (() => {
+                modelManager.addModelFiles([concertoModel, modelBase, farm2fork, modelBase]);
+            }).should.throw(/namespace already exists/);
+        });
+
+        it('should return an error for duplicate namespace from objects', () => {
+            let mf1 = sinon.createStubInstance(ModelFile);
+            mf1.getNamespace.returns('org.doge');
+            let mf2 = sinon.createStubInstance(ModelFile);
+            mf2.getNamespace.returns('org.doge.base');
+            modelManager.addModelFiles([mf1,mf2]);
+            (() => {
+                modelManager.addModelFiles([mf1]);
+            }).should.throw(/namespace already exists/);
+        });
+
     });
 
     describe('#updateModelFile', () => {