What does it mean when a girl does a double take, then waves and smiles?

I was running on a treadmill the other day at my gym, and this girl I know(albeit not too well) walks by and does a double take, then she smiled and waved to me. She always does this, but not always with the double take. She never initiates conversation, but she seems to go out of her way to get my attention. Is she into me, or is she just being nice?
What does it mean when a girl does a double take, then waves and smiles?
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