Rails の学習を開始しましたが、Rails を Mac にインストールする際に問題が発生しました。私のバージョンは現在 OS X 10.8.3 で、問題は次のとおりです。
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:230:in `activate': can't activate rack (~> 1.4.5, runtime) for ["actionpack-3.2.13", "railties-3.2.13"], already activated rack-1.5.2 for ["rack-ssl-1.3.3", "railties-3.2.13"] (Gem::LoadError)
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:246:in `activate'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `each'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `activate'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:246:in `activate'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `each'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:245:in `activate'
from /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems.rb:1056:in `gem'
from /usr/bin/rails:18
この問題を解決してレールを正しく実行するにはどうすればよいですか?