Add unique option

Use unique option to remove dulidate result:

    autocomplete :item, :brand, full: true, unique: true

lib/rails-jquery-autocomplete/orm/active_record.rb

@@ -29,6 +29,10 @@ def active_record_get_autocomplete_items(parameters)
             limit(limit).order(order)
         items = items.where(where) unless where.blank?
 
+        if options[:unique]
+          scopes <<  lambda { unscope(:select).select("MIN(#{table_name}.#{model.primary_key}) as #{model.primary_key}, #{table_name}.#{method}").group(method) }
+        end
+
         scopes.each do |scope|
           items = case scope
                   when String
@@ -42,16 +46,17 @@ def active_record_get_autocomplete_items(parameters)
       end
 
       def get_autocomplete_select_clause(model, method, options)
-        if sqlite?
+        base = if sqlite?
           table_name = model.quoted_table_name
-          ([
-              "#{table_name}.#{model.connection.quote_column_name(model.primary_key)} as #{model.primary_key}",
-              "#{table_name}.#{model.connection.quote_column_name(method)} as #{method}"
-            ] + (options[:extra_data].blank? ? [] : options[:extra_data]))
+          [
+            "#{table_name}.#{model.connection.quote_column_name(model.primary_key)} as #{model.primary_key}",
+            "#{table_name}.#{model.connection.quote_column_name(method)} as #{method}"
+          ]
         else
           table_name = model.table_name
-          (["#{table_name}.#{model.primary_key}", "#{table_name}.#{method}"] + (options[:extra_data].blank? ? [] : options[:extra_data]))
+          ["#{table_name}.#{model.primary_key}", "#{table_name}.#{method}"]
         end
+        base + ( options[:extra_data] || [] )
       end
 
       def get_autocomplete_where_clause(model, term, method, options)