Ignore multiple port declarations

Right now if you specify a port via `-p $PORT` and via a `config/puma.rb` then puma will incorrectly try to bind to the same port twice and will fail.

This PR calls `uniq!` on the array to remove duplicate ports from the `binds` array.

lib/puma/configuration.rb

@@ -64,6 +64,8 @@ module Puma
       unless @options[:tag]
         @options[:tag] = infer_tag
       end
+
+      @options[:binds].uniq!
     end
 
     def infer_tag

test/config/app.rb

@@ -1,3 +1,5 @@
+port ENV['PORT'] if ENV['PORT']
+
 app do |env|
   [200, {}, ["embedded app"]]
 end

test/test_config.rb

@@ -14,6 +14,17 @@ class TestConfigFile < Test::Unit::TestCase
     assert_equal [200, {}, ["embedded app"]], app.call({})
   end
 
+  def test_double_bind_port
+    port = rand(30_000..40_000).to_s
+    with_env("PORT" => port) do
+      opts = { :binds => ["tcp://#{Configuration::DefaultTCPHost}:#{port}"], :config_file => "test/config/app.rb"}
+      conf = Puma::Configuration.new opts
+      conf.load
+
+      assert_equal ["tcp://0.0.0.0:#{port}"], conf.options[:binds]
+    end
+  end
+
   def test_lowleve_error_handler_DSL
     opts = { :config_file => "test/config/app.rb" }
     conf = Puma::Configuration.new opts
@@ -23,4 +34,19 @@ class TestConfigFile < Test::Unit::TestCase
 
     assert_equal [200, {}, ["error page"]], app.call({})
   end
+
+  private
+
+    def with_env(env = {})
+      original_env = {}
+      env.each do |k, v|
+        original_env[k] = ENV[k]
+        ENV[k] = v
+      end
+      yield
+    ensure
+      original_env.each do |k, v|
+        ENV[k] = v
+      end
+    end
 end