I was just reading this on an online max-DPR build. There were some who thought using the Metamagic option Twinned Spell on the warlock's Eldritch Blast cantrip was illegal, and some who thought the opposite.
The Twinned Spell option for the sorcerer's Metamagic feature reads (PHB, page 102; emphasis mine)
When you cast a spell that targets only one creature and doesn't have a range of self, you can spend a number of sorcery points equal to the spell's level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).
Can Twinned Spell be used with Eldritch Blast?