Show the duplicate warning for similar links
The duplicate topic warning wasn't shown when the same article is posted again with a slightly different URL, for example, http://utf8everywhere.org/ versus https://utf8everywhere.org/.
Example: https://tildes.net/~comp/nuy/utf_8_everywhere
(Note: after posting the topic, someone edited the link to point to https.)
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information