diff --git a/lib/hamlit/attribute.rb b/lib/hamlit/attribute.rb
index 8bde517c..2d26058d 100644
--- a/lib/hamlit/attribute.rb
+++ b/lib/hamlit/attribute.rb
@@ -62,7 +62,7 @@ module Hamlit
           when :id
             result[key] = [base[key], target[key]].compact.join('_')
           when :class
-            result[key] = [base[key], target[key]].flatten.compact.map(&:to_s).sort.join(' ')
+            result[key] = [base[key], target[key]].flatten.compact.map(&:to_s).sort.uniq.join(' ')
           end
         else
           result[key] = base[key].nil? ? target[key] : base[key]
diff --git a/spec/hamlit/engine/old_attributes_spec.rb b/spec/hamlit/engine/old_attributes_spec.rb
index 02026787..ed8cc8d3 100644
--- a/spec/hamlit/engine/old_attributes_spec.rb
+++ b/spec/hamlit/engine/old_attributes_spec.rb
@@ -124,9 +124,11 @@ describe Hamlit::Engine do
       it 'joins attribute class and element class' do
         assert_render(<<-HAML, <<-HTML, compatible_only: :haml)
           .foo{ class: ['bar'] }
+          .foo{ class: ['bar', 'foo'] }
           .foo{ class: ['bar', nil] }
           .foo{ class: ['bar', 'baz'] }
         HAML
+          <div class='bar foo'></div>
           <div class='bar foo'></div>
           <div class='bar foo'></div>
           <div class='bar baz foo'></div>