diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -1,7 +1,7 @@
 class Problem < ActiveRecord::Base
 
   belongs_to :description
-  has_and_belongs_to_many :contests
+  has_and_belongs_to_many :contests, :uniq => true
   has_many :test_pairs, :dependent => :delete_all
 
   validates_presence_of :name