Remove unique option from migration

db/migrate/20160301174731_add_fingerprint_index.rb

 class AddFingerprintIndex < ActiveRecord::Migration
   def change
-    add_index :keys, :fingerprint, unique: false
+    add_index :keys, :fingerprint
   end
 end

db/schema.rb

@@ -514,7 +514,7 @@ ActiveRecord::Schema.define(version: 20160316124047) do
   end
 
   add_index "keys", ["created_at", "id"], name: "index_keys_on_created_at_and_id", using: :btree
-  add_index "keys", ["fingerprint"], name: "index_keys_on_fingerprint", unique: true, using: :btree
+  add_index "keys", ["fingerprint"], name: "index_keys_on_fingerprint", using: :btree
   add_index "keys", ["user_id"], name: "index_keys_on_user_id", using: :btree
 
   create_table "label_links", force: :cascade do |t|