Adds a prefix to fix the issue

lib/gitlab/sidekiq_middleware/duplicate_jobs/client.rb

@@ -5,7 +5,7 @@ module Gitlab
     module DuplicateJobs
       class Client
         def call(worker_class, job, queue, _redis_pool, &block)
-          DuplicateJob.new(job, queue).schedule(&block)
+          ::Gitlab::SidekiqMiddleware::DuplicateJobs::DuplicateJob.new(job, queue).schedule(&block)
         end
       end
     end