core/boxes: Do not update the same box twice

Vagrant's environment (which includes the known list of boxes and
versions) is established at the start of the Vagrant run. This means
that box downloads which occur during the run are not contained in the
set until the next run. This causes duplicate box downloads to raise an
error in multi-machine Vagrantfiles.

This commit fixes that issue by pre-processing the machines by provider
and version, creating a unique set of boxes to update.

Fixes GH-6042

plugins/commands/box/command/update.rb

@@ -86,6 +86,8 @@ module VagrantPlugins
         end
 
         def update_vms(argv, provider, download_options)
+          machines = {}
+
           with_target_vms(argv, provider: provider) do |machine|
             if !machine.config.vm.box
               machine.ui.output(I18n.t(
@@ -100,6 +102,14 @@ module VagrantPlugins
               next
             end
 
+            name     = machine.box.name
+            provider = machine.box.provider
+            version  = machine.config.vm.box_version || machine.box.version
+
+            machines["#{name}_#{provider}_#{version}"] = machine
+          end
+
+          machines.each do |_, machine|
             box = machine.box
             version = machine.config.vm.box_version
             # Get download options from machine configuration if not specified