My son keeps calling me by my first name: Heather or more like "Head-er"

Dominic is at this really annoying stage where he basically copies anything he hears. He hears Justin call me by my name all the time and he follows suit. He'll run down the hallway and say, "Heather (sounds more like Head-er), mommy, Head-er." I'll turn around and correct him and say, "Don't call me Head-er. Call me mommy." And-he'll say, "Okay, Head-er." What's up with that? I don't hear him calling Justin by his first name. Why does he have to call me by my first name? Doesn't he understand that I went through childbirth and pain for him??? Just joking... It's actually pretty funny!