Check for exisiting deduplication_key index before removing

If you run the migration from the previous commit and there
are exisiting non-unique records, the migration will fail.

The exisiting index will be removed, but when you run
migrations again they will fail because the index can
no longer be removed.

db/migrate/20191105165518_add_uniqueness_constraint_to_pre_ingest_work_deduplication_key.rb

@@ -0,0 +1,6 @@
+class AddUniquenessConstraintToPreIngestWorkDeduplicationKey < 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: true
+  end
+end