diff --git a/process.c b/process.c
index be583be6e3..ba800b9058 100644
--- a/process.c
+++ b/process.c
@@ -6136,6 +6136,19 @@ maxgroups(void)
  *
  *     Process.groups   #=> [27, 6, 10, 11]
  *
+ *  Note that this method is just a wrapper of getgroups(2).
+ *  This means that the following characteristics of
+ *  the results are completely depends on your system:
+ *
+ *  - the result is sorted
+ *  - the result includes effective GIDs
+ *  - the result does not include duplicated GIDs
+ *
+ *  You can certainly get a sorted unique GID list of
+ *  the current process by this expression:
+ *
+ *     Process.groups.unique.sort
+ *
  */
 
 static VALUE
diff --git a/spec/ruby/core/process/groups_spec.rb b/spec/ruby/core/process/groups_spec.rb
index 331c53939a..325deb5977 100644
--- a/spec/ruby/core/process/groups_spec.rb
+++ b/spec/ruby/core/process/groups_spec.rb
@@ -6,8 +6,8 @@ describe "Process.groups" do
       groups = `id -G`.scan(/\d+/).map { |i| i.to_i }
       gid = Process.gid
 
-      expected = (groups.sort - [gid]).sort
-      actual = (Process.groups - [gid]).sort
+      expected = (groups.sort - [gid]).uniq.sort
+      actual = (Process.groups - [gid]).uniq.sort
       actual.should == expected
     end
   end