README.txt

# bundle exec を使うと期待した通りに動作する
$ bundle install --path vendor/bundle
$ bundle exec cap -s server=myserver sample_stage sample
  * 02:58:34 == Currently executing `sample_stage'
[:server, "myserver"]
#<Capistrano::Role:0x00000100c4e090 @static_servers=[], @dynamic_servers=[]>
    triggering start callbacks for `sample'
  * 02:58:34 == Currently executing `multistage:ensure'
  * 02:58:34 == Currently executing `sample'
[:server, "myserver"]
#<Capistrano::Role:0x00000100c4de10 @static_servers=[myserver], @dynamic_servers=[]>

# bundle exec 経由でないと sample_stage.rb が2回ロードされる
$ cap -s server=myserver sample_stage sample
    triggering load callbacks
  * 03:00:44 == Currently executing `sample_stage'
[:server, "default-server"]
#<Capistrano::Role:0x00000100c3c228 @static_servers=[], @dynamic_servers=[]>
  * 03:00:44 == Currently executing `sample_stage'
[:server, "myserver"]
#<Capistrano::Role:0x00000100c3c228 @static_servers=[], @dynamic_servers=[]>
    triggering start callbacks for `sample'
  * 03:00:44 == Currently executing `multistage:ensure'
  * 03:00:44 == Currently executing `sample'
[:server, "myserver"]
#<Capistrano::Role:0x00000100c45260 @static_servers=[default-server, myserver], @dynamic_servers=[]>
                                                     ↑結果として server にデフォルト値が追加されてしまう

Capfile

load 'deploy' if respond_to?(:namespace)

load 'config/deploy'

require "capistrano_colors"

set :stages, %w(sample_stage)
set :default_stage, "sample_stage"

require 'capistrano/ext/multistage'

deploy.rb

desc "sample task"
task :sample do
  p [:server, fetch(:server)]
  p roles[:app]
end

Gemfile

# -*- coding: utf-8 -*-
source 'https://rubygems.org'

gem 'capistrano'
gem 'capistrano_colors'
gem 'capistrano-ext'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    capistrano (2.13.3)
      highline
      net-scp (>= 1.0.0)
      net-sftp (>= 2.0.0)
      net-ssh (>= 2.0.14)
      net-ssh-gateway (>= 1.1.0)
    capistrano-ext (1.2.1)
      capistrano (>= 1.0.0)
    capistrano_colors (0.5.5)
    highline (1.6.14)
    net-scp (1.0.4)
      net-ssh (>= 1.99.1)
    net-sftp (2.0.5)
      net-ssh (>= 2.0.9)
    net-ssh (2.5.2)
    net-ssh-gateway (1.1.0)
      net-ssh (>= 1.99.1)

PLATFORMS
  ruby

DEPENDENCIES
  capistrano
  capistrano-ext
  capistrano_colors

sample_stage.rb

server = fetch(:server, "default-server")
p [:server, server]
p roles[:server_each]
#roles[:app].clear
role :app, server