diff --git a/lib/jasmine-core/jasmine.js b/lib/jasmine-core/jasmine.js
index 14ab310e..0dc988b7 100644
--- a/lib/jasmine-core/jasmine.js
+++ b/lib/jasmine-core/jasmine.js
@@ -3345,9 +3345,9 @@ getJasmineRequireObj().Configuration = function(j$) {
      * error.
      * @name Configuration#forbidDuplicateNames
      * @type boolean
-     * @default false
+     * @default true
      */
-    forbidDuplicateNames: false,
+    forbidDuplicateNames: true,
     /**
      * Whether to issue warnings for certain deprecated functionality
      * every time it's used. If not set or set to false, deprecation warnings
diff --git a/spec/core/ConfigurationSpec.js b/spec/core/ConfigurationSpec.js
index 849dbf90..b90ec59c 100644
--- a/spec/core/ConfigurationSpec.js
+++ b/spec/core/ConfigurationSpec.js
@@ -25,7 +25,7 @@ describe('Configuration', function() {
     expect(subject.specFilter()).toEqual(true);
     expect(subject.hideDisabled).toEqual(false);
     expect(subject.autoCleanClosures).toEqual(true);
-    expect(subject.forbidDuplicateNames).toEqual(false);
+    expect(subject.forbidDuplicateNames).toEqual(true);
     expect(subject.verboseDeprecations).toEqual(false);
     expect(subject.detectLateRejectionHandling).toEqual(false);
   });
diff --git a/spec/core/EnvSpec.js b/spec/core/EnvSpec.js
index db258a00..353ab285 100644
--- a/spec/core/EnvSpec.js
+++ b/spec/core/EnvSpec.js
@@ -487,14 +487,14 @@ describe('Env', function() {
 
       it('does not throw an error when called in a describe', function() {
         env.setParallelLoadingState('helpers');
-        check();
+        check(1);
         env.setParallelLoadingState('specs');
-        check();
+        check(2);
 
-        function check() {
+        function check(disambiguator) {
           let done = false;
 
-          env.describe('a suite', function() {
+          env.describe('a suite ' + disambiguator, function() {
             expect(function() {
               env.it('a spec');
               env.beforeAll(function() {});
@@ -594,14 +594,14 @@ describe('Env', function() {
 
       it('does not throw an error when called in a describe', function() {
         env.setParallelLoadingState('helpers');
-        check();
+        check(1);
         env.setParallelLoadingState('specs');
-        check();
+        check(2);
 
-        function check() {
+        function check(disambiguator) {
           let done = false;
 
-          env.describe('a suite', function() {
+          env.describe('a suite ' + disambiguator, function() {
             expect(function() {
               env.it('a spec');
               env.afterAll(function() {});
diff --git a/spec/core/integration/SpecRunningSpec.js b/spec/core/integration/SpecRunningSpec.js
index a1975876..06ae0552 100644
--- a/spec/core/integration/SpecRunningSpec.js
+++ b/spec/core/integration/SpecRunningSpec.js
@@ -40,7 +40,7 @@ describe('spec running', function() {
         env.it('should run nested suites', function() {
           foo++;
         });
-        env.it('should run nested suites', function() {
+        env.it('should run nested suites 2', function() {
           bar++;
         });
       });
diff --git a/src/core/Configuration.js b/src/core/Configuration.js
index 82e30327..1d3a509c 100644
--- a/src/core/Configuration.js
+++ b/src/core/Configuration.js
@@ -95,9 +95,9 @@ getJasmineRequireObj().Configuration = function(j$) {
      * error.
      * @name Configuration#forbidDuplicateNames
      * @type boolean
-     * @default false
+     * @default true
      */
-    forbidDuplicateNames: false,
+    forbidDuplicateNames: true,
     /**
      * Whether to issue warnings for certain deprecated functionality
      * every time it's used. If not set or set to false, deprecation warnings