Autocomplete: duplicates of existing terms are suggested (regression)

#675446-238:
There appears to a major regression in the committed code: duplicates are suggested!

Steps to reproduce:

1. Make sure a free tagging field already has the "foo" and "bar" values. Assume these are the only two terms in the system.
2. Now go back to the node and edit, type a comma and "b".
3. Expected: nothing is suggested. Reality: "bar" is suggested again.

This problem was in fact implicitly being demonstrated in #230.