"But to Hannah he would give a double portion, for the face" [reading apayim as a double portion] (1 Samuel 1:5). "For the face" means: with a cheerful and welcoming countenance. The double portion was set against Peninnah and her children: one portion corresponding to all the children [that Peninnah had]. Another interpretation: it was the finest portion among them.
Why Elkanah Gave Hannah a Double Portion
Yalkut Shimoni on Nach 77:8
ולחנה יתן מנה אחת אפים בכפלים. אפים בסבר פנים יפות כנגד פנינה וילדיה כנגד כל הבנים. דבר אחר היפה שבהם: