Full Idea
The utilitarian argument for equality assumes that people have 'diminishing marginal returns' for goods. If there are two people and two nice chocolate biscuits, then utilitarianism is likely to recommend one each.
Gist of Idea
Utilitarians argue for equal distribution because of diminishing utility of repetition
Source
Jonathan Wolff (An Introduction to Political Philosophy (Rev) [2006], 5 'Arguments')
Book Reference
Wolff,Jonathan: 'An Introduction to Political Philosophy (Rev)' [OUP 2006], p.151
A Reaction
The point is that the second biscuit provides slightly diminished pleasure. This is why you can buy boxes of assorted biscuits, which you are then not required to share.