Repeatedly adding a new contributor leads to multiple autocomplete entries for that contributor

[skytreader]

Steps:

1. In a new form, create a book with a new contributor. Submit book.
2. In the same form, create another book from contributor but, this time, make a mistake when entering the contributor's name. (For example: instead of entering "Sapkowski, Andrzej", just enter "Sapkowski, Andr".)
3. Complete the second book anyway.
4. Check the contributor field for autocomplete entries for the contributor concerned and see duplicates.

[skytreader]

Note: Seems to happen even if it is not a completely new contributor; merely adding the same contributor in one session is sufficient to trigger the bug. You don't even need to "make a mistake".