Hash table collisions: why use a linked list if we can use a hash set?
An hash set is an hash table.  Using an hash set to handle collisions in an hash table is equivalent to using a bigger hash table, with an hashing function which is a combination of the hashing functions of both level.

In other words, you'd probably be better with a bigger initial table (for instance there is no risk of resonance between the two hash functions which could lead to an higher collision rate than expected at the second level).