diff --git a/activerecord/lib/active_record/calculations.rb b/activerecord/lib/active_record/calculations.rb
index c3e7313f6d85aec237201187e2876bfebacf3a8a..490c8f47127bb0579177f5967b6177de5481b02b 100644
--- a/activerecord/lib/active_record/calculations.rb
+++ b/activerecord/lib/active_record/calculations.rb
@@ -169,7 +169,10 @@ def construct_calculation_sql(operation, column_name, options) #:nodoc:
             end
           end
 
-          sql = "SELECT #{operation}(#{'DISTINCT ' if options[:distinct]}#{column_name}) AS #{aggregate_alias}"
+          if options[:distinct] && column_name.to_s !~ /\s*DISTINCT\s+/i
+            distinct = 'DISTINCT ' 
+          end
+          sql = "SELECT #{operation}(#{distinct}#{column_name}) AS #{aggregate_alias}"
 
           # A (slower) workaround if we're using a backend, like sqlite, that doesn't support COUNT DISTINCT.
           sql = "SELECT COUNT(*) AS #{aggregate_alias}" if use_workaround
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index 1771065a667c2141887784ab4f7d055bff5383f4..0ed40b469d3a2ff51d07585af20104745b4f975b 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -241,6 +241,10 @@ def test_should_count_selected_field_with_include
     assert_equal 4, Account.count(:distinct => true, :include => :firm, :select => :credit_limit)
   end
 
+  def test_should_count_manual_select_with_include
+    assert_equal 6, Account.count(:select => "DISTINCT accounts.id", :include => :firm)
+  end
+
   def test_count_with_column_parameter
     assert_equal 5, Account.count(:firm_id)
   end