[wvxvw]

var xml:XML = 
<response>
    <item>
        <id>1</id>
        <cost>150</cost>
        <period>1</period>
    </item>
    <item>
        <id>1</id>
        <cost>100</cost>
        <period>2</period>
    </item>
    <item>
        <id>2</id>
        <cost>70</cost>
        <period>1</period>
    </item>
     <item>
         <id>2</id>
         <cost>90</cost>
        <period>2</period>
     </item>
</response>;
var d:Dictionary = new Dictionary();
 
var list:XMLList = xml.item.id.(!d[valueOf().text().toString()] && 
				(d[valueOf().text().toString()] = true));
trace(list.toXMLString());
/*
<id>1</id>
<id>2</id>
*/