Ensure puzzle uniqueness per contestant validation error is shown

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"

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"