Steps to reproduce:

1. Add a field of type ezkeyword to the 'article' content type;
2. Create and publish an article with a lowercase keyword, i.e 'foo';
3. Create and publish a second article with the same keyword, but capitalised 'Foo';
4. Edit the second article;
5. Notice that the list of keywords is "foo,Foo", should only be "Foo";

Note: according to the customer, this is the relevant piece of code in ezpublish-kernel:

https://github.com/ezsystems/ezpublish-kernel/commit/b5d8178660eb2824d387b9c64217b1d9fa9ef2e6#commitcomment-20961740