diff --git a/lib/pbbuilder.rb b/lib/pbbuilder.rb
index 18a7358..44d46ec 100644
--- a/lib/pbbuilder.rb
+++ b/lib/pbbuilder.rb
@@ -75,6 +75,7 @@ def set!(field, *args, &block)
         if arg.respond_to?(:to_hash)
           # example syntax that should end up here:
           #   pb.fields {"one" => "two"}
+
           arg.to_hash.each { |k, v| @message[name][k] = v }
         elsif arg.respond_to?(:to_ary) && !descriptor.type.eql?(:message)
           # pb.fields ["one", "two"]
@@ -88,11 +89,13 @@ def set!(field, *args, &block)
         else
           # example syntax that should end up here:
           #   pb.fields "one"
-          @message[name].push arg
+
+          @message[name].replace Array.wrap(arg)
         end
       else
         # example syntax that should end up here:
         #   pb.field "value"
+        
         @message[name] = arg
       end
     else
diff --git a/test/pbbuilder_test.rb b/test/pbbuilder_test.rb
index fca62eb..ba86f1b 100644
--- a/test/pbbuilder_test.rb
+++ b/test/pbbuilder_test.rb
@@ -25,7 +25,7 @@ class PbbuilderTest < ActiveSupport::TestCase
 
     assert_equal "Hello world", person.name
     assert_equal "Friend #1", person.friends.first.name
-    assert_equal ["ok", "that's", "cool"], person.field_mask.paths
+    assert_equal ["cool"], person.field_mask.paths
     assert_equal "Manuelo", person.best_friend.name
     assert_equal "Eggs", person.favourite_foods["Breakfast"]
   end
@@ -45,6 +45,17 @@ class PbbuilderTest < ActiveSupport::TestCase
 
     assert_equal(["ok", "that's"], p.field_mask.paths)
   end
+  
+  test "Would not stack duplicated values, uses last one defined" do
+    person = Pbbuilder.new(API::Person.new) do |pb|
+      pb.field_mask do
+        pb.paths ["ok", "that's"]
+        pb.paths ["cool"]
+      end
+    end.target!
+
+    assert_equal ["cool"], person.field_mask.paths
+  end
 
   test "it can extract fields in a nice way" do
     klass = Struct.new(:name)