Esigns,
исполнение RegExp по заявке )))
<script type="text/javascript">
var str = "email5@email.com email1@email.com email2@email.com email1@email.com email2@email.com email3@email.com email1@email.com email1@email.com email2@email.com email1@email.com email2@email.com email3@email.com email5@email.com";
myRe = /(\S+@\S+)(?![\s\S]+?\1)/gm;
alert(str.match(myRe).join("\n"));
</script>