patch.diff

diff --git a/lib/Mojo/IOLoop/Resolver.pm b/lib/Mojo/IOLoop/Resolver.pm
index a0f1770..bbd2e4f 100644
--- a/lib/Mojo/IOLoop/Resolver.pm
+++ b/lib/Mojo/IOLoop/Resolver.pm
@@ -76,6 +76,8 @@ my $DNS_TYPES = {
 # "localhost"
 our $LOCALHOST = '127.0.0.1';
 
+my $ONGOING = {};
+
 sub DESTROY { shift->_cleanup }
 
 sub build {
@@ -123,6 +125,18 @@ sub lookup {
   return $self->ioloop->defer(sub { $self->$cb($LOCALHOST) })
     if $name eq 'localhost';
 
+  my $cbs = $ONGOING->{$name};
+  if (defined $cbs) {
+    push $cbs, $cb;
+    return;
+  }
+
+  $ONGOING->{$name} = [$cb];
+
+  my $done = sub {
+    $self->$_(@_) for @{delete $ONGOING->{$name}};
+  };
+
   # Resolve
   $self->resolve(
     $name => A => sub {
@@ -130,15 +144,15 @@ sub lookup {
 
       # IPv4
       my $result = first { $_->[0] eq 'A' } @$records;
-      return $self->$cb($result->[1]) if $result;
+      return $done->($result->[1]) if $result;
 
       # IPv6
       $self->resolve(
         $name => AAAA => sub {
           my ($self, $records) = @_;
-          return $self->$cb()
+          return $done->()
             unless my $result = first { $_->[0] eq 'AAAA' } @$records;
-          $self->$cb($result->[1]);
+          $done->($result->[1]);
         }
       );
     }

t.pl

use Mojo::UserAgent;
use v5.10;

my $u = Mojo::UserAgent->new;

sub yay {
  say "START";
  $u->get('http://google.no/', sub {
    say "DONE";
  });
}

yay;yay;yay;

Mojo::IOLoop->start;