The base rate is 8% for double and 3% for triple. It shows up on a few pages on the wiki, most of which I don't remember. But I'm looking at the awakening page and it does mention the base rate. (it does so because it points out that awakening raises double and triple rates to 10% and 5% respectively).
Actually, the buff/debuff effects page should also mention it, in the double attack section.