avoid unique validation errors switch to upserting subjects

app/services/import/subject.rb

@@ -10,9 +10,25 @@ def initialize(zooniverse_subject_id, context)
     end
 
     def run
-      subject = ::Subject.find_or_create_by!(context_id: context.id, zooniverse_subject_id: zooniverse_subject_id)
+      subject = upsert_subject
       SubjectBackfillerJob.perform_async(subject.id)
       subject
     end
+
+    private
+
+    def upsert_subject
+      # use upsert to avoid collisions for inserting rows
+      upsert_results = ::Subject.upsert_all(
+        [{
+          context_id: context.id,
+          zooniverse_subject_id: zooniverse_subject_id
+        }],
+        unique_by: %i[zooniverse_subject_id context_id],
+        update_only: [:context_id] # must have an update clause to get a result set
+      )
+      upserted_subject_id = upsert_results.to_a.first['id']
+      ::Subject.find(upserted_subject_id)
+    end
   end
 end