It's rude to decline a genuine gift.
Abigail's grandmother, Helen, just brought her a new doll, but she already has the same exact one.
Abigail wants to respond to the present.
Abigail tells Helen that she loves it, and doesn't mention she already owned this one.
Abigail tells Helen, no thank you, citing she already has this one.
Helen feels good about herself for making her granddaughter happy.
Helen is stuck with the doll and now feels sad that she disappointed Abigail.