diff --git a/Library/Homebrew/cmd/update.rb b/Library/Homebrew/cmd/update.rb
index 70d3812504..504d4115b1 100644
--- a/Library/Homebrew/cmd/update.rb
+++ b/Library/Homebrew/cmd/update.rb
@@ -83,7 +83,8 @@ module Homebrew
       migration = TAP_MIGRATIONS[f]
       next unless migration
       tap = Tap.fetch(*migration.split("/"))
-      tap.install
+      tap.install unless tap.installed?
+
       # update tap for each Tab
       tabs = dir.subdirs.map { |d| Tab.for_keg(Keg.new(d)) }
       next if tabs.first.source["tap"] != "Homebrew/homebrew"