diff --git a/TemplateData.php b/TemplateData.php
index 30085ffc..c83a5af0 100644
--- a/TemplateData.php
+++ b/TemplateData.php
@@ -107,6 +107,7 @@ $wgResourceModules['ext.templateDataGenerator.ui'] = array(
 	'messages' => array(
 		'comma-separator',
 		'templatedata-doc-no-params-set',
+		'templatedata-exists-on-related-page',
 		'templatedata-modal-button-add-language',
 		'templatedata-modal-button-addparam',
 		'templatedata-modal-button-apply',
@@ -157,7 +158,7 @@ $wgResourceModules['ext.templateDataGenerator.ui'] = array(
 		'templatedata-modal-title-paramorder',
 		'templatedata-modal-title-templatedesc',
 		'templatedata-modal-title-templateparam-details',
-		'templatedata-modal-title-templateparams',
+		'templatedata-modal-title-templateparams'
 	)
 );
 
diff --git a/i18n/en.json b/i18n/en.json
index 497cee92..2cd1741b 100644
--- a/i18n/en.json
+++ b/i18n/en.json
@@ -28,6 +28,7 @@
 	"templatedata-doc-params": "Template parameters",
 	"templatedata-editbutton": "Manage TemplateData",
 	"templatedata-errormsg-jsonbadformat": "Bad JSON format. Either correct it, or delete the current <templatedata> tags and try again.",
+	"templatedata-exists-on-related-page": "Please note: there is already a TemplateData block on the related page '[[$1]]'.",
 	"templatedata-helplink": "Information about TemplateData",
 	"templatedata-helplink-target": "//www.mediawiki.org/wiki/Special:MyLanguage/Help:TemplateData",
 	"templatedata-invalid-duplicate-value": "Property \"$1\" (\"$3\") is a duplicate of \"$2\".",
diff --git a/i18n/qqq.json b/i18n/qqq.json
index f6b1661e..9e7ab3c6 100644
--- a/i18n/qqq.json
+++ b/i18n/qqq.json
@@ -35,6 +35,7 @@
 	"templatedata-doc-params": "Used as caption for the table which has the following headings:\n* {{msg-mw|Templatedata-doc-param-name}}\n* {{msg-mw|Templatedata-doc-param-desc}}\n* {{msg-mw|Templatedata-doc-param-default}}\n* {{msg-mw|Templatedata-doc-param-status}}\n{{Identical|Template parameter}}",
 	"templatedata-editbutton": "The label of the button to manage templatedata, appearing above the editor field.",
 	"templatedata-errormsg-jsonbadformat": "Error message that appears in case the JSON string is not possible to parse. The user is asked to either correct the json syntax or delete the values between the &lt;templatedata&gt; tags and try again.",
+	"templatedata-exists-on-related-page": "A messaged that is displayed when there already is a templatedata string in a related page. $1: The page where templatedata already exists.",
 	"templatedata-helplink": "The label of the link to the TemplateData documentation, appearing above the editor field.",
 	"templatedata-helplink-target": "{{ignore}} The target of the link to the TemplateData documentation",
 	"templatedata-invalid-duplicate-value": "Displayed when an array that must only contain unique values contains a duplicate.\n* $1 - name of property containing the duplicate\n* $2 - name of property with first occurrence of value\n* $3 - the value being duplicated",
diff --git a/modules/ext.templateDataGenerator.data.js b/modules/ext.templateDataGenerator.data.js
index bd4fa94a..bd11b6e4 100644
--- a/modules/ext.templateDataGenerator.data.js
+++ b/modules/ext.templateDataGenerator.data.js
@@ -61,17 +61,28 @@ OO.mixinClass( mw.TemplateData.Model, OO.EventEmitter );
 /**
  * Get information from the mediaWiki API
  * @param {string} page Page name
+ * @param {boolean} [getTemplateData] Fetch the templatedata in the page.
  * @return {jQuery.Promise} API promise
  */
-mw.TemplateData.Model.static.getApi = function ( page ) {
-	var api = new mw.Api();
-	return api.get( {
-		action: 'query',
-		prop: 'revisions',
-		rvprop: 'content',
-		indexpageids: '1',
-		titles: page
-	} );
+mw.TemplateData.Model.static.getApi = function ( page, getTemplateData ) {
+	var config,
+		api = new mw.Api(),
+		baseConfig = {
+			action: getTemplateData ? 'templatedata' : 'query',
+			titles: page
+		};
+	if ( getTemplateData ) {
+		config = $.extend( baseConfig, {
+			redirects: '1'
+		} );
+	} else {
+		config = $.extend( baseConfig, {
+			prop: 'revisions',
+			rvprop: 'content',
+			indexpageids: '1'
+		} );
+	}
+	return api.get( config );
 };
 
 /**
diff --git a/modules/ext.templateDataGenerator.editPage.js b/modules/ext.templateDataGenerator.editPage.js
index 6703b643..580d6602 100644
--- a/modules/ext.templateDataGenerator.editPage.js
+++ b/modules/ext.templateDataGenerator.editPage.js
@@ -8,19 +8,32 @@
 	'use strict';
 
 	$( function () {
-		var config = {
+		var pieces, isDocPage,
+			pageName = mw.config.get( 'wgPageName' ),
+			config = {
+				pageName: pageName,
 				isPageSubLevel: false
 			},
-			$textbox = $( '#wpTextbox1' ),
-			pageName = mw.config.get( 'wgPageName' );
+			$textbox = $( '#wpTextbox1' );
 
 		// Check if there's an editor textarea and if we're in the proper namespace
 		if ( $textbox.length && mw.config.get( 'wgCanonicalNamespace' ) === 'Template' ) {
-			if ( pageName.indexOf( '/' ) > -1 ) {
-				config.parentPage = pageName.substr( 0, pageName.indexOf( '/' ) );
-				config.isPageSubLevel = pageName.indexOf( '/' ) > -1;
-			}
+			pieces = pageName.split( '/' );
+			isDocPage = pieces.length > 1 && pieces[ pieces.length - 1 ] === 'doc';
 
+			config = {
+				pageName: pageName,
+				isPageSubLevel: pieces.length > 1,
+				parentPage: pageName,
+				isDocPage: isDocPage
+			};
+
+			// Only if we are in a doc page do we set the parent page to
+			// the one above. Otherwise, all parent pages are current pages
+			if ( isDocPage ) {
+				pieces.pop();
+				config.parentPage = pieces.join( '/' );
+			}
 			// Prepare the editor
 			mw.libs.tdgUi.init( $( '#mw-content-text' ), $textbox, config );
 		}
diff --git a/modules/ext.templateDataGenerator.ui.js b/modules/ext.templateDataGenerator.ui.js
index 5ca52519..0c886cb7 100644
--- a/modules/ext.templateDataGenerator.ui.js
+++ b/modules/ext.templateDataGenerator.ui.js
@@ -10,6 +10,8 @@
 	'use strict';
 	mw.libs.tdgUi = ( function () {
 		var isPageSubLevel,
+			isDocPage,
+			pageName,
 			parentPage,
 			$textbox,
 			/**
@@ -28,13 +30,20 @@
 			 * Display error message in the edit window
 			 * @param {string} msg Message to display
 			 * @param {string} type Message type 'notice' or 'error'
+			 * @param {boolean} [parseHTML] The message should be parsed
 			 */
-			setNoticeMessage: function ( msg, type ) {
+			setNoticeMessage: function ( msg, type, parseHTML ) {
 				type = type || 'error';
 				editNoticeLabel.$element.toggleClass( 'errorbox', type === 'error' );
 
+				if ( parseHTML ) {
+					// OOUI's label elements do not parse strings and display them
+					// as-is. If the message contains html that should be parsed,
+					// we have to transform it into a jQuery object
+					msg = $( '<span>' ).append( $.parseHTML( msg ) );
+				}
 				editNoticeLabel.setLabel( msg );
-				editNoticeLabel.$element.show();
+				editNoticeLabel.toggle( true );
 			},
 
 			/**
@@ -42,7 +51,7 @@
 			 */
 			resetNoticeMessage: function () {
 				editNoticeLabel.setLabel( '' );
-				editNoticeLabel.$element.hide();
+				editNoticeLabel.toggle( false );
 			}
 		},
 
@@ -114,13 +123,15 @@
 			 *  current wikitext from.
 			 */
 			init: function ( $container, $editorTextbox, userConfig ) {
-				var editHelpButtonWidget,
+				var editHelpButtonWidget, relatedPage,
 					config = userConfig;
 
 				$textbox = $editorTextbox;
 
+				pageName = config.pageName;
 				parentPage = config.parentPage;
 				isPageSubLevel = !!config.isPageSubLevel;
+				isDocPage = !!config.isDocPage;
 
 				editOpenDialogButton = new OO.ui.ButtonWidget( {
 					label: mw.msg( 'templatedata-editbutton' )
@@ -144,6 +155,26 @@
 				tdgDialog = new mw.TemplateData.Dialog( config );
 				windowManager.addWindows( [ tdgDialog ] );
 
+				// Check if there's already a templatedata in a related page
+				relatedPage = isDocPage ? parentPage : pageName + '/doc';
+				editOpenDialogButton.setDisabled( true );
+				mw.TemplateData.Model.static.getApi( relatedPage, true )
+					.then( function ( result ) {
+						var msg;
+						if ( !$.isEmptyObject( result.pages ) ) {
+							// HACK: Setting a link in the messages doesn't work. The bug report offers
+							// a somewhat hacky work around that includes setting a separate message
+							// to be parsed.
+							// https://phabricator.wikimedia.org/T49395#490610
+							msg = mw.message( 'templatedata-exists-on-related-page', relatedPage ).plain();
+							mw.messages.set( { 'templatedata-string-exists-hack-message': msg } );
+							msg = mw.message( 'templatedata-string-exists-hack-message' ).parse();
+
+							editArea.setNoticeMessage( msg, 'error', true );
+						}
+						editOpenDialogButton.setDisabled( false );
+					} );
+
 				// Prepend to container
 				$container
 					.prepend(