fix exercise_file duplication bug

app/models/code_ocean/file.rb

@@ -7,6 +7,7 @@ module CodeOcean
     def validate(record)
       existing_files = File.where(name: record.name, path: record.path, file_type_id: record.file_type_id,
                                   context_id: record.context_id, context_type: record.context_type).to_a
+
       unless existing_files.empty?
         if (not record.context.is_a?(Exercise)) || (record.context.new_record?)
           record.errors[:base] << 'Duplicate'

app/models/exercise.rb

@@ -35,13 +35,13 @@ class Exercise < ApplicationRecord
   validates :public, boolean_presence: true
   validates :title, presence: true
   validates :token, presence: true, uniqueness: true
+  validates_uniqueness_of :uuid
 
   @working_time_statistics = nil
   attr_reader :working_time_statistics
 
   MAX_EXERCISE_FEEDBACKS = 20
 
-
   def average_percentage
     if average_score and maximum_score != 0.0 and submissions.exists?(cause: 'submit')
       (average_score / maximum_score * 100).round(2)