Optionally enforce uniqueness of spec and suite names

This is off by default for backwards compatibility but can be enabled
by setting the forbidDuplicateNames env config property to true.

Fixes #1633.

src/core/Env.js

@@ -138,6 +138,15 @@ getJasmineRequireObj().Env = function(j$) {
        * @default true
        */
       autoCleanClosures: true,
+      /**
+       * Whether to forbid duplicate spec or suite names. If set to true, using
+       * the same name multiple times in the same immediate parent suite is an
+       * error.
+       * @name Configuration#forbidDuplicateNames
+       * @type boolean
+       * @default false
+       */
+      forbidDuplicateNames: false,
       /**
        * Whether or not to issue warnings for certain deprecated functionality
        * every time it's used. If not set or set to false, deprecation warnings
@@ -186,7 +195,8 @@ getJasmineRequireObj().Env = function(j$) {
         'hideDisabled',
         'stopOnSpecFailure',
         'stopSpecOnExpectationFailure',
-        'autoCleanClosures'
+        'autoCleanClosures',
+        'forbidDuplicateNames'
       ];
 
       booleanProps.forEach(function(prop) {

src/core/Suite.js

@@ -251,6 +251,16 @@ getJasmineRequireObj().Suite = function(j$) {
     );
   };
 
+  Suite.prototype.hasChildWithDescription = function(description) {
+    for (const child of this.children) {
+      if (child.description === description) {
+        return true;
+      }
+    }
+
+    return false;
+  };
+
   Object.defineProperty(Suite.prototype, 'metadata', {
     get: function() {
       if (!this.metadata_) {

src/core/SuiteBuilder.js

@@ -161,6 +161,8 @@ getJasmineRequireObj().SuiteBuilder = function(j$) {
         j$.util.validateTimeout(timeout);
       }
 
+      this.checkDuplicate_(description, 'spec');
+
       const spec = this.specFactory_(description, fn, timeout, filename);
       if (this.currentDeclarationSuite_.markedExcluding) {
         spec.exclude();
@@ -170,7 +172,27 @@ getJasmineRequireObj().SuiteBuilder = function(j$) {
       return spec;
     }
 
+    checkDuplicate_(description, type) {
+      if (!this.env_.configuration().forbidDuplicateNames) {
+        return;
+      }
+
+      if (this.currentDeclarationSuite_.hasChildWithDescription(description)) {
+        const parentDesc =
+          this.currentDeclarationSuite_ === this.topSuite
+            ? 'top suite'
+            : `"${this.currentDeclarationSuite_.getFullName()}"`;
+        throw new Error(
+          `Duplicate ${type} name "${description}" found in ${parentDesc}`
+        );
+      }
+    }
+
     suiteFactory_(description, filename) {
+      if (this.topSuite) {
+        this.checkDuplicate_(description, 'suite');
+      }
+
       const config = this.env_.configuration();
       const parentSuite = this.currentDeclarationSuite_;
       const reportedParentSuiteId =