Cannot remove duplicate terms from Applicable Terms list

In the "Applicable Taxonomy Terms" block, if you accidentally add a duplicate term it can't be easily removed. Take the following, for example:

• Purple Drazi [tid:42]
• Green Drazi [tid:43]
• Purple Drazi [tid:42]
• Vorlon [tid:1]

If you try to remove the second Purple Drazi by erasing the text from that field, when you go back to the edit page again it will still be there.

What should happen is the following:

• When saving the list it should not allow duplicates. Given there's no way of doing this at the schema level it needs to be done in code.
• For any existing records, editing the list should filter duplicates.
• It might be worth adding an update script to fix existing records.