Do multiple copies of Ezreal Prodigy on board stack their cost reduction effects, or does the cost reduction only apply once regardless of how many copies are present?

Ruling: Multiple copies of Ezreal Prodigy stack their cost reduction effects. Each Ezreal applies its cost reduction once.

Nuances:

• Riftbound has no Legend Rule for any card except those with the Unique keyword, so multiple copies of the same unit can exist on board simultaneously