fix(export/job): race condition when export was already generated

app/jobs/export_job.rb

@@ -4,6 +4,8 @@ class ExportJob < ApplicationJob
   discard_on ActiveRecord::RecordNotFound
 
   def perform(export)
+    return if export.generated?
+
     export.compute_with_safe_stale_for_purge do
       export.compute
     end