install: fix if isn't migrated

Library/Homebrew/cmd/install.rb

@@ -75,14 +75,15 @@ module Homebrew
 
         if f.installed?
           msg = "#{f.full_name}-#{f.installed_version} already installed"
-          unless f.linked_keg.symlink?
-            if f.oldname && (HOMEBREW_CELLAR/f.oldname).exist?
-              msg << ", it's just not migrated"
-            elsif !f.keg_only?
-              msg << ", it's just not linked"
-            end
-          end
+          msg << ", it's just not linked" unless f.linked_keg.symlink? || f.keg_only?
           opoo msg
+        elsif f.oldname && (dir = HOMEBREW_CELLAR/f.oldname).exist? && !dir.subdirs.empty? \
+            && f.tap == Tab.for_keg(dir.subdirs.first).tap && !ARGV.force?
+          # Check if the formula we try to install is the same as installed
+          # but not migrated one. If --force passed then install anyway.
+          opoo "#{f.oldname} already installed, it's just not migrated"
+          puts "You can migrate formula with `brew migrate #{f}`"
+          puts "Or you can force install it with `brew install #{f} --force`"
         else
           formulae << f
         end