Quote Originally Posted by Ualaa View Post

But I've found Rip will randomly fail to apply on top of another Rip (even from the same Druid), with the message to the effect that it cannot overwrite the existing spell.
This may happen if you used a trinket or some other ability to increase the damage of the first rip. The second rip would be less powerful (do less damage over time) and therefore fails to be applied. I had the same problem when using thorns, with a message something like "a more powerful spell is active"