A bonus is something given in addition to what is expected. So, because added is already included in the meaning of bonus, added bonus is a redundancy.
Examples
In these examples, the adjective added could be removed with no loss of meaning:
The director is as fun and dynamic as the movies, so her talk should be an added bonus. [Washington Post]
For those whose politics lean to the left, there’s also the added bonus of a plot that suggests the Bush administration fabricated the existence of WMD in Iraq to instigate a war. [Wall Street Journal]
As an extra added bonus, the PS3 version also includes a digital copy of Henry Wadsworth Longfellow’s translation of the original poem. [Syracuse.com]
