Avoid adding index if already exists

db/migrate/20160301174731_add_fingerprint_index.rb

@@ -4,6 +4,7 @@ class AddFingerprintIndex < ActiveRecord::Migration
 
   DOWNTIME = false
 
+  # https://gitlab.com/gitlab-org/gitlab-ee/issues/764
   def change
     args = [:keys, :fingerprint]
 
@@ -11,6 +12,6 @@ class AddFingerprintIndex < ActiveRecord::Migration
       args << { algorithm: :concurrently }
     end
 
-    add_index(*args)
+    add_index(*args) unless index_exists?(:keys, :fingerprint)
   end
 end