Código HTML:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<SCRIPT LANGUAGE="JavaScript">
function mifuncion(form){
temp = form.combo.value
if (form.caja.value==""){
form.caja.value = temp;
}else{
form.caja.value = form.caja.value + "\n" + temp;
}
}
</SCRIPT>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<form name="form" method="post" action="">
<p>
<select name="combo" >
<option value="Valor1">Valor1</option>
<option value="Valor2">Valor2</option>
<option value="Valor3">Valor3</option>
<option value="Valor4">Valor4</option>
<option value="Valor5">Valor5</option>
<option value="Valor6">Valor6</option>
</select>
<label>
<input type="button" name="Submit" value=">>" onClick="mifuncion(form)">
</label>
<textarea name="caja"></textarea>
</p>
<p> </p>
</form>
</body>
</html>
Evitar la repetición ya es un poco mas complicado necesitas un array cotejar si ya exite ...
Quim