An anonymous user (not logged in) tries to submit a CD for which there already is a stub:
http://test.musicbrainz.org/cdtoc/attach?toc=1+16+222320+150+16215+25125+35580+46585+60183+73655+80547+93496+105385+123534+134759+153434+170985+188375+208210&tracks=16&id=UvrBqkLXMfu.beyEFy2AGvnxy20- (example, not specific)

As the user is not logged in the Server reports:

Lookup CD
CD Not Found

MusicBrainz does not have any CDs with the TOC:
[...]

The user is given the option to "Submit this CD using the simple method.", basically adding the same CD stub that is already in the database again.
It is not mentioned, that there is already a stub in the database for this disc.

The problem is the same in current stable (musicbrainz.org), current beta (76dbf14) and test (a1960b3).

Logging in shows the CD stub and gives the option to import it.