Hi
Duplicate Detection would only work during uploading of assets.
For Content Fragment, we are actually creating it (not uploading), so detection would not work.
For here is a quick workaround for you, duplicate detection checks dam.sha1 (It is Hash created for an asset), so when we upload another asset again sha1 hash is created for that asset and then sha1 for Assetes are compared, if they are same, then error pops up.
So For Content Fregment as well, sha1 is created, so you can create a custom service which check sha1 and notify you if sha1 matches.
Where to find sha1:-
/content/dam/cf_test-1/jcr:content/metadata
I hope this will help you.
Thanks and Regards
Kautuk Sahni