diff --git a/app/models/completion.rb b/app/models/completion.rb
index 8468002..9bbf682 100644
--- a/app/models/completion.rb
+++ b/app/models/completion.rb
@@ -36,7 +36,8 @@ class Completion < ApplicationRecord
   before_save :add_time_seconds
 
   validates :display_time_from_start, presence: true, format: { with: /\A(((\d\d|\d):\d\d|\d\d|\d):\d\d|\d\d|\d)\z/ }
-  validates :puzzle_id, uniqueness: { scope: :contestant }
+  validates :contestant_id, uniqueness: { scope: :puzzle }, if: -> { contest.puzzles.size == 1 }
+  validates :puzzle_id, uniqueness: { scope: :contestant }, if: -> { contest.puzzles.size > 1 }
 
   def add_time_seconds
     arr = display_time_from_start.split(":")
diff --git a/config/locales/en.yml b/config/locales/en.yml
index dd2c423..52e467f 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -82,6 +82,8 @@ en:
       models:
         completion:
           attributes:
+            contestant_id:
+              taken: "This contestant has already completed the puzzle"
             display_time_from_start:
               blank: Mandatory
               invalid: "Allowed formats: xx:xx:xx, x:xx:xx, xx:xx, x:xx, xx"
diff --git a/config/locales/fr.yml b/config/locales/fr.yml
index bf55770..35f15f5 100644
--- a/config/locales/fr.yml
+++ b/config/locales/fr.yml
@@ -53,6 +53,8 @@ fr:
       models:
         completion:
           attributes:
+            contestant_id:
+              taken: "Ce.tte participant.e a déjà complété le puzzle"
             display_time_from_start:
               blank: Obligatoire
               invalid: "Formats autorisés: xx:xx:xx, x:xx:xx, xx:xx, x:xx, xx"