Address feedback

services/web/app/coffee/Features/AccountMerge/AccountMergeEmailController.coffee

@@ -17,33 +17,20 @@ module.exports = AccountMergeEmailController =
 				return res.status(404).send()
 			if data.origin != 'sl'
 				logger.log {}, "Only sharelatex origin supported"
-				# TODO: better error handling
 				return res.status(501).send()
 			{ sl_id, v1_id, final_email } = data
 			UserGetter.getUser sl_id, {_id: 1, overleaf: 1}, (err, user) ->
 				return next(err) if err?
 				if !user?
 					logger.err {userId: sl_id}, 'SL user not found for account-merge'
-					# TODO: better error handling
 					return res.status(400).send()
 				if user?.overleaf?.id?
 					logger.err {userId: sl_id}, 'SL user is already linked to overleaf'
 					return res.status(400).send()
 				logger.log {sl_id, v1_id, final_email, origin: data.origin},
 					"[AccountMergeEmailController] about to merge sharelatex and overleaf-v1 accounts"
-				UserUpdater.updateUser sl_id, {
-					'$set': {
-						'overleaf.id': v1_id,
-						email: final_email
-					},
-					'$push': {
-						emails: {
-							email: final_email, createdAt: new Date()
-						}
-					}
-				}, (err) ->
+				UserUpdater.mergeSharelatexAndV1Accounts sl_id, v1_id, final_email, (err) ->
 					return next(err) if err?
 					res.render 'account_merge/finish', {
-						finalEmail: final_email,
-						continueUrl: "#{Settings.accountMerge.sharelatexHost}/user/login_to_ol_v2"
+						finalEmail: final_email
 					}

services/web/app/coffee/Features/User/UserUpdater.coffee

@@ -185,11 +185,28 @@ module.exports = UserUpdater =
 					return callback(new Errors.NotFoundError('user id and email do no match'))
 				FeaturesUpdater.refreshFeatures userId, true, callback
 
+	mergeSharelatexAndV1Accounts: (sl_id, v1_id, final_email, callback) ->
+		UserGetter.getUser {'overleaf.id': v1_id}, {_id: 1}, (err, otherUser) =>
+			if otherUser?
+				return callback(new Error('v1_id matches an existing user overleaf.id, cannot merge accounts'))
+			@updateUser sl_id, {
+				'$set': {
+					'overleaf.id': v1_id,
+					email: final_email
+				},
+				'$push': {
+					emails: {
+						email: final_email, createdAt: new Date()
+					}
+				}
+			}, callback
+
 [
 	'updateUser'
 	'changeEmailAddress'
 	'setDefaultEmailAddress'
 	'addEmailAddress'
 	'removeEmailAddress'
+	'mergeSharelatexAndV1Accounts'
 ].map (method) ->
 	metrics.timeAsyncMethod(UserUpdater, method, 'mongo.UserUpdater', logger)

services/web/app/views/account_merge/finish.pug

@@ -22,6 +22,7 @@ block content
 										| with your merged account:
 
 									p.text-center
-										a.btn.btn-lg.btn-success(href=continueUrl) Continue Logging In
+										a.btn.btn-lg.btn-success(href=settings.accountMerge.sharelatexHost+'/user/login_to_ol_v2')
+											| Continue Logging In
 										br
 										a.btn-small(href="/login") or go to the login page