diff --git a/NEWS b/NEWS
index 45570f1b7..266f96a72 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,7 @@
 
 Enhancements
  - [web] avoid saving an empty calendar name
+ - [web] prohibit duplicate contact categories in Preferences module
 
 Bug fixes
  - [web] properly handle Windows-1256 charaset (#4781)
diff --git a/UI/PreferencesUI/English.lproj/Localizable.strings b/UI/PreferencesUI/English.lproj/Localizable.strings
index 2b75b93f2..56019e7dd 100644
--- a/UI/PreferencesUI/English.lproj/Localizable.strings
+++ b/UI/PreferencesUI/English.lproj/Localizable.strings
@@ -192,6 +192,7 @@
 /* Contact */
 "Personal Address Book" = "Personal Address Book";
 "Collected Address Book" = "Collected Address Book";
+"Contact categories must have unique names." = "Contact categories must have unique names.";
 
 /* IMAP Accounts */
 "Settings" = "Settings";
diff --git a/UI/Templates/PreferencesUI/UIxPreferences.wox b/UI/Templates/PreferencesUI/UIxPreferences.wox
index ea4f0431c..db4e53927 100644
--- a/UI/Templates/PreferencesUI/UIxPreferences.wox
+++ b/UI/Templates/PreferencesUI/UIxPreferences.wox
@@ -509,8 +509,12 @@
 	          <input type="text"
                          label:aria-label="Contact Category"
                          required="required"
+                         name="contactCategory_{{$index}}"
                          ng-model="app.preferences.defaults.SOGoContactsCategories[$index]"
                          sg-focus-on="contactCategory_{{$index}}"/>
+                    <div ng-messages="preferencesForm['contactCategory_' + $index].$error">
+                      <div ng-message="duplicate"><var:string label:value="Duplicate name"/></div>
+                    </div>
                 </md-input-container>
                 <md-button type="button" class="sg-icon-button"
                            label:aria-label="Remove Contact Category"
diff --git a/UI/WebServerResources/js/Preferences/PreferencesController.js b/UI/WebServerResources/js/Preferences/PreferencesController.js
index 1d79b4fa5..135075b00 100644
--- a/UI/WebServerResources/js/Preferences/PreferencesController.js
+++ b/UI/WebServerResources/js/Preferences/PreferencesController.js
@@ -54,10 +54,14 @@
     };
 
     this.addCalendarCategory = function(form) {
-      this.preferences.defaults.SOGoCalendarCategories.push(l('New category'));
-      this.preferences.defaults.SOGoCalendarCategoriesColorsValues.push("#aaa");
-      focus('calendarCategory_' + (this.preferences.defaults.SOGoCalendarCategories.length - 1));
-      form.$setDirty();
+      var i = _.indexOf(this.preferences.defaults.SOGoCalendarCategories, l('New category'));
+      if (i < 0) {
+        this.preferences.defaults.SOGoCalendarCategories.push(l('New category'));
+        this.preferences.defaults.SOGoCalendarCategoriesColorsValues.push("#aaa");
+        form.$setDirty();
+        i = this.preferences.defaults.SOGoCalendarCategories.length - 1;
+      }
+      focus('calendarCategory_' + i);
     };
 
     this.resetCalendarCategoryValidity = function(index, form) {
@@ -376,6 +380,20 @@
         });
       }
 
+      // Contact categories must be unique
+      if (this.preferences.defaults.SOGoContactsCategories.length !=
+          _.uniq(this.preferences.defaults.SOGoContactsCategories).length) {
+        Dialog.alert(l('Error'), l("Contact categories must have unique names."));
+        _.forEach(this.preferences.defaults.SOGoContactsCategories, function (value, i, keys) {
+          if (form['contactCategory_' + i].$dirty &&
+              (keys.indexOf(value) != i ||
+               keys.indexOf(value, i+1) > -1)) {
+            form['contactCategory_' + i].$setValidity('duplicate', false);
+            sendForm = false;
+          }
+        });
+      }
+
       if (sendForm)
         return this.preferences.$save().then(function(data) {
           if (!options || !options.quick) {