When A Means Zero

A candidate writes the character mapping as c - 'a' instead of c - 'a' + 1:

hash_value = (hash_value + (c - 'a') * p_pow) % m;   // a -> 0, b -> 1, ...

with p = 31, m = 1e9 + 9, and the exponent growing with the index. Which pair of distinct strings does this version give the same hash?