Illusionist's Bracers

From Guildmasters' Guide to Ravnica, page 178.

Description
Wondrous item, very rare (requires attunement by a spellcaster)

A powerful illusionist of House Dimir originally developed these bracers, which enabled her to create multiple Minor Illusions at once. The bracers' power, though, extends far beyond illusions.

While wearing the bracers, whenever you cast a cantrip, you can use a bonus action on the same turn to cast that cantrip a second time.