diff --git a/plugins/commands/up/start_mixins.rb b/plugins/commands/up/start_mixins.rb
index 8be96c68a..069ceb843 100644
--- a/plugins/commands/up/start_mixins.rb
+++ b/plugins/commands/up/start_mixins.rb
@@ -1,3 +1,5 @@
+require "set"
+
 module VagrantPlugins
   module CommandUp
     module StartMixins
@@ -27,14 +29,14 @@ module VagrantPlugins
       # This validates the provisioner flags and raises an exception
       # if there are invalid ones.
       def validate_provisioner_flags!(options, argv)
-        provisioner_instance_names = []
+        provisioner_names = Set.new
         with_target_vms(argv) do |machine|
-          provisioner_instance_names << machine.config.vm.provisioners.map(&:name)
+          machine.config.vm.provisioners.map(&:name).each do |name|
+            provisioner_names.add(name)
+          end
         end
 
-        provisioner_instance_names.flatten!.uniq!
-
-        if (provisioner_instance_names & options[:provision_types]).empty?
+        if (provisioner_names & options[:provision_types]).empty?
           (options[:provision_types] || []).each do |type|
               klass = Vagrant.plugin("2").manager.provisioners[type]
               if !klass