If you are looking for no code solution I don't believe it is possible.
Only workaround I can think of is to write some custom webservice that will get your data, parse it from comma separated values and then use it as external data source to populate choices in listbox.
I used back on 2007 similar technique when working with no code browser forms.
Original answer - opposite solution
You can easily put selected Multiselection Listbox values to text field by using this formula:
eval(eval(field1[. != ""]; 'concat(., ", ")'); "..")
(where field1 is name of your listbox binded repeating field)
You can set some other field to this value by using rule when you submit your form or on button click. After that you can propagate this field to list - but it really depends on your infopath form logic.
I don't quite understand why you need to bind only unique values since if your listbox contains same values when you select one you will select all items with same value. However if that is the case formula above will get only one.
You can find some more info on InfoPathDev Forum here.