multiselect_single

This module provides a simple feature to allow you to configure an unlimited cardinality entity reference field, use the Select display but have it behave like multiple widgets instead of a single select element you have to Control + Click on to get multiple selections.

This is better UX for users who want the ability to select from simple lists (< 100 items) who don't like autocomplete inputs and without manipulating the ugly browser version of multiselect. It requires nothing more than setting the form field widget under Manage Form Display. This is intended for non-taxonomy or non-hierarchical lists. Client Side Hierarchical Select or similar modules are suited for that.

This has been lightly tested but does work with unlimited cardinality versions of an element.