Remove uniqueness constraint from deduplication_key

Adding a unique constraint to the `deduplication_key` solved
grouping for an initial import, but it causes problems when
you re-import. After a re-import `PreIngestFile`s are added to
the existing `PreIngestWork`s which are assoicated with existing
`CsvImportDetail`s.

db/migrate/20191105211127_remove_uniqueness_constraint_to_pre_ingest_work_deduplication_key.rb

@@ -0,0 +1,7 @@
+class RemoveUniquenessConstraintToPreIngestWorkDeduplicationKey < ActiveRecord::Migration[5.1]
+  def change
+    remove_index :zizia_pre_ingest_works, :deduplication_key if index_exists?(:zizia_pre_ingest_works, :deduplication_key)
+
+    add_index :zizia_pre_ingest_works, :deduplication_key, unique: false
+  end
+end

spec/dummy/db/schema.rb

@@ -605,7 +605,7 @@
     t.datetime "updated_at", null: false
     t.string "deduplication_key"
     t.index ["csv_import_detail_id"], name: "index_zizia_pre_ingest_works_on_csv_import_detail_id"
-    t.index ["deduplication_key"], name: "index_zizia_pre_ingest_works_on_deduplication_key", unique: true
+    t.index ["deduplication_key"], name: "index_zizia_pre_ingest_works_on_deduplication_key"
   end
 
 end