Rookie question.
{Yeva's Forcemage} names itself in its enters-the-battlefield (ETB) ability. Maybe lots of cards do, but I'm new.
If one YF is on the battlefield from a previous turn, and a second one enters, do I get the ability twice because the second one is named on itself and on the first one too?
No, as the card does not say, "Name a card; cards of the chosen name get +2/+2" when it enters the battlefield, you target one creature on the field, and it gets the bonus.
RESOLVED
When a card names itself, it refers to that specific card only, not another one. For instance, if you had {Thragtusk} on the battlefield and you play another one, you don't gain 10 life; you gain 5 life.