Grammar for Everyone - Barbara Dykes 2007

Double comparatives
Introduction
Part II The parts of speech

The suffix ’er’ and the preceding qualifier ’more’ have the same meaning but we choose the most suitable for the context. We cannot use both together.

not He is more hungrier than a wolf.

but He is hungrier than a wolf (is).

or He is more hungry than a wolf.

Similarly, with superlatives:

not We saw the most skinniest boy.

but We saw the skinniest boy.

Similarly, we should avoid using comparative adjectives such as more or very in addition to words that carry a superlative meaning within themselves.

not It was a very unique show.

but It was a unique show.