Adding duplicates tag shouldn't be allowed

Wiki pages, like the Sandbox for example (on my freshly installed XWiki Enterprise 2.2.4.28034) allow to add duplicates tags without a "conflict" error.

To reproduce the bug, you should :
1. go to a wiki page (the sandbox for example)
2. on the bottom, click on "[+] add tags" link
3. under the "tag" field, input "test,test,test" ; then validate the form

The page prints 3 times the tag "test"