Your baby's loved faces from the get-go, so it's no wonder she's drawn to the one she sees in the mirror -- long before she realizes who it is.
She'll be able to raise her head for a glimpse in an unbreakable crib mirror at around 4 months old. This is also great exercise to strengthen the muscles she'll use for sitting and crawling.
Once she can sit up in front of a mirror (at about 6 months), she may babble to her image or try to touch it. She'll delight in how her identical playmate responds and makes eye contact. To play too:
Place a few mirrored tiles next to her changing area.
Visit different mirrors throughout the house together and talk about what you see.
Hold a teddy bear in front of a mirror and watch her look back and forth between them.
While looking at your reflections, point to your baby and say her name, then point to yourself and say "Mommy." But don't expect her to figure out who's who right away: She won't pick up that she's looking at herself until she's about 7 months old.