It's not really a bug as much as an artifact of the REPL. You are seeing what the call to (link ...) prints and then the return value of the same call printed right after it by the REPL.
Changing the return value illustrates the point:
arc> (no (link "A Link" "somepage.html"))
<a href="somepage.html">A Link</a>nil
It might not be optimal that link (or really tag) always returns "</", but I'm not sure it matters.