I tried applying Shredded to the Regenerating Test Dummy by using Viper's Fangs. This ability applies 12sec of Shredded.
If I then try to refresh the Shredded duration WHILE IT IS ABOVE 10sec using "Chained Kunai: Fang of the Dragon" or by using "Eviscerate: Messy" it will refresh the duration to 12sec, working as intended.
However, if I try to refresh the duration WHILE IT IS BELOW 10sec, it will only add a few seconds to the Shredded debuff.
When I hit the Regenerating Test Dummy with my fully charged Chained Kunai and at the moment of impact Shredded was at 6sec, the duration got upped to 8sec (while it should have been upped to 16sec).
When I hit the Regenerating Test Dummy with a fully charged Chained Kunai (or Eviscerate) while it was at 4sec, it sometimes added 1 second, sometimes 2, sometimes 0 (the latter might be a lag issue though).
I haven't tested if this is the same for the advantages (nor abilities) that refresh the duration of any other debuffs.