Undefined in 404s logs

Hi,

I can find lots of entries in my 404s log ending with /undefined.
The referrer is always the same url of the entry, without /undefined.
For example:
Referrer: http://www.ottimizzazione-pc.it/capire-quali-cartelle-e-file-occupano-piu-spazio-nel-computer/
URL flagged: http://www.ottimizzazione-pc.it/capire-quali-cartelle-e-file-occupano-piu-spazio-nel-computer/undefined

What does that mean? How should I fix this?

A few more posts with the same questions haven’t been answered, such as
http://wordpress.org/support/topic/plugin-redirection-the-word-undefined-appended-to-redirect-log-entries

http://wordpress.org/plugins/redirection/