Hmm, this is a tough one.
On the one hand, Dunkit is right, insofar as some people will think the double expression bad style or too colloquial. (In this respect it's a bit like the English "off of", as in "take the hat off of your head"...)
On the other hand, Dunkit's 'corrected' version may sound more correct, but it's definitely less common. With all the other litte indicators of register (elision of 'e', usage of 'mal', second person), this sounds like a very colloquial situation and I'd expect colloquial usage. Interestingly, the corrected version actually sounds truncated in this light. Note that 'drauf' is the colloquial version of 'darauf', which can't be used here.
I'm going out on a limb here, but maybe the addition of 'drauf' is an unconscious attempt to replace a participle that's preceived as missing. For example, in a formal version of Dunkit's example I'd expect 'gespeichert' or something similar before the 'ist'. Perhaps we are so used to having 'auf' followed by some further clarification that we automatically add an empty 'drauf' if there is none? Any thoughts?
In reply to your actual question:
No, I don't think there's a real reason to add the 'drauf'. For most people, it adds nothing to the sentence's meaning. Oh, and forget the translation with the additional 'thereon' - 'auf... drauf' simply translates to 'on' or maybe 'on top' in an actually spatial context.