Double comparatives are adjectives with more than one comparative marker. For example, the comparative word more and the comparative suffix -er are both applied to the adjective loud in the phrase more louder in the sentence above. Like comparatives, superlatives (most, -est) may also be doubled.
Double comparatives may also involve two instances of the suffix -er, as in the word louderer, though the double usage of this suffix might be a distinct phenomenon from the usage of more and -er together. In particular, there is typically no difference in meaning between louder and more louder, while it may be that there is a difference in meaning between louder and louderer.
a. The Duke of Milan / and his more braver daughter could controul thee.
(Shakespeare, The Tempest)
b. This was the most unkindest cut of all.
(Shakespeare, Julius Caesar)
Additional comparisons?
Nevins (2012:92) cites Radford (1977) for the claim that double comparatives do not license two comparisons. For example, the following sentence contains a double comparative and feature two comparisons (comparison with Bill and comparison with you), and they are unacceptable to many native speakers because of this:
2) John is more taller than Bill than you are (you are taller than Bill but not as much as John).
Kennedy and McNally (2005), however, provide examples like (3), with less, which do seem to license an extra layer of comparison:
3) an old department store a lot less taller than the city hall building than is the new company headquarters.
Seuren (1972:561) also suggests that double comparatives with more can in fact license two comparisons:
4) John is more taller than Bill than Peter. (John is taller than Bill, but he is even taller than Peter)
Less co-occurring with -er
Corver (2005) notes that the -er affix doesn’t always mean “more” but can be realized in forms like less happier. He gives the following example from Shakespeare:
5) Or as a moat defensive to a house,
Against the envy of less happier lands,
This blessed plot, this earth, this realm, this England.
(Shakespeare, King Richard II, ii, i)
https://ygdp.yale.edu/phenomena/double-comparatives#who-says-this