diff --git a/app/models/topic.rb b/app/models/topic.rb
index de5ea80d0d4..bfec8ac390a 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -582,6 +582,8 @@ class Topic < ActiveRecord::Base
     if private_message?
       # If the user exists, add them to the message.
       user = User.find_by_username_or_email(username_or_email)
+      raise StandardError.new I18n.t("topic_invite_user_exists") if user.present? && topic_allowed_users.where(user_id: user.id).exists?
+
       if user && topic_allowed_users.create!(user_id: user.id)
         # Create a small action message
         add_small_action(invited_by, "invited_user", user.username)
@@ -605,6 +607,8 @@ class Topic < ActiveRecord::Base
     else
       # invite existing member to a topic
       user = User.find_by_username(username_or_email)
+      raise StandardError.new I18n.t("topic_invite_user_exists") if user.present? && topic_allowed_users.where(user_id: user.id).exists?
+
       if user && topic_allowed_users.create!(user_id: user.id)
         # rate limit topic invite
         RateLimiter.new(invited_by, "topic-invitations-per-day", SiteSetting.max_topic_invitations_per_day, 1.day.to_i).performed!
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 841cf5108ff..6e7d5c342c0 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -138,6 +138,8 @@ en:
   reading_time: "Reading time"
   likes: "Likes"
 
+  topic_invite_user_exists: "Sorry, that user has already been invited. You may only invite a user to a topic once."
+
   too_many_replies:
     one: "We're sorry, but new users are temporarily limited to 1 reply in the same topic."
     other: "We're sorry, but new users are temporarily limited to %{count} replies in the same topic."