Avoid adding index if already exists

CHANGELOG-EE

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.10.0 (unreleased)
   - Rename Git Hooks to Push Rules
+  - Fix EE keys fingerprint add index migration if came from CE
 
 v 8.9.5
   - Fix of quoted text in lock tooltip. !518

db/migrate/20160301174731_add_fingerprint_index.rb

@@ -2,6 +2,7 @@
 class AddFingerprintIndex < ActiveRecord::Migration
   disable_ddl_transaction!
 
+  # https://gitlab.com/gitlab-org/gitlab-ee/issues/764
   def change
     args = [:keys, :fingerprint]
 
@@ -9,6 +10,6 @@ class AddFingerprintIndex < ActiveRecord::Migration
       args << { algorithm: :concurrently }
     end
 
-    add_index(*args)
+    add_index(*args) unless index_exists?(:keys, :fingerprint)
   end
 end