diff --git a/lib/mocha.js b/lib/mocha.js
index aab5604448..c0a5aa6dd9 100644
--- a/lib/mocha.js
+++ b/lib/mocha.js
@@ -106,6 +106,10 @@ function Mocha(options) {
     options.color = 'color' in options ? options.color : options.useColors;
   }
 
+  // Globals are passed in as options.global, with options.globals for backward compatibility.
+  options.globals = options.global || options.globals || [];
+  delete options.global;
+
   this.grep(options.grep)
     .fgrep(options.fgrep)
     .ui(options.ui)
@@ -540,7 +544,7 @@ Mocha.prototype._growl = growl.notify;
  * Specifies whitelist of variable names to be expected in global scope.
  *
  * @public
- * @see {@link https://mochajs.org/#--globals-names|CLI option}
+ * @see {@link https://mochajs.org/#-global-variable-name|CLI option}
  * @see {@link Mocha#checkLeaks}
  * @param {String[]|String} globals - Accepted global variable name(s).
  * @return {Mocha} this
@@ -551,9 +555,12 @@ Mocha.prototype._growl = growl.notify;
  * mocha.globals(['jQuery', 'MyLib']);
  */
 Mocha.prototype.globals = function(globals) {
-  this.options.globals = (this.options.globals || [])
+  this.options.globals = this.options.globals
     .concat(globals)
-    .filter(Boolean);
+    .filter(Boolean)
+    .filter(function(elt, idx, arr) {
+      return arr.indexOf(elt) === idx;
+    });
   return this;
 };
 
diff --git a/test/unit/mocha.spec.js b/test/unit/mocha.spec.js
index 16fbbd7f82..ed99a3e257 100644
--- a/test/unit/mocha.spec.js
+++ b/test/unit/mocha.spec.js
@@ -21,6 +21,7 @@ describe('Mocha', function() {
       sandbox.stub(Mocha.prototype, 'useColors').returnsThis();
       sandbox.stub(utils, 'deprecate');
       sandbox.stub(Mocha.prototype, 'timeout').returnsThis();
+      sandbox.stub(Mocha.prototype, 'globals').returnsThis();
     });
 
     describe('when "useColors" option is defined', function() {
@@ -64,6 +65,44 @@ describe('Mocha', function() {
         );
       });
     });
+
+    describe('when "options.global" is provided', function() {
+      it('should pass "options.global" to #globals()', function() {
+        // eslint-disable-next-line no-new
+        new Mocha({global: ['singular']});
+        expect(Mocha.prototype.globals, 'to have a call satisfying', [
+          ['singular']
+        ]).and('was called once');
+      });
+      it('should delete mocha.options.global', function() {
+        var mocha = new Mocha({global: ['singular']});
+        expect(mocha.options.global, 'to be', undefined);
+      });
+    });
+
+    describe('when "options.globals" is provided', function() {
+      it('should pass "options.globals" to #globals()', function() {
+        // eslint-disable-next-line no-new
+        new Mocha({globals: ['plural']});
+        expect(Mocha.prototype.globals, 'to have a call satisfying', [
+          ['plural']
+        ]).and('was called once');
+      });
+    });
+
+    describe('when "options.global" AND "options.globals" are provided', function() {
+      it('should pass "options.global" to #globals(), ignoring "options.globals"', function() {
+        // eslint-disable-next-line no-new
+        new Mocha({global: ['singular'], globals: ['plural']});
+        expect(Mocha.prototype.globals, 'to have a call satisfying', [
+          ['singular']
+        ]).and('was called once');
+      });
+      it('should delete mocha.options.global', function() {
+        var mocha = new Mocha({global: ['singular'], globals: ['plural']});
+        expect(mocha.options.global, 'to be', undefined);
+      });
+    });
   });
 
   describe('#allowUncaught()', function() {
@@ -159,6 +198,7 @@ describe('Mocha', function() {
     describe('when argument is valid', function() {
       var elem = 'foo';
       var elem2 = 'bar';
+      var elem3 = 'baz';
 
       it('should add string to the whitelist', function() {
         var mocha = new Mocha(opts);
@@ -174,6 +214,14 @@ describe('Mocha', function() {
         expect(mocha.options.globals, 'to contain', elem, elem2);
         expect(mocha.options.globals, 'to have length', elems.length);
       });
+
+      it('should not have duplicates', function() {
+        var mocha = new Mocha({globals: [elem, elem2]});
+        var elems = [elem, elem2, elem3];
+        mocha.globals(elems);
+        expect(mocha.options.globals, 'to contain', elem, elem2, elem3);
+        expect(mocha.options.globals, 'to have length', elems.length);
+      });
     });
   });