L.R.
You didn't go to the dentist yet, right? Let the dentist advise you, not what you read online. This may indeed be an extra tooth -- or it may be one of her "regular" baby teeth that is erupting in the wrong position. The dentist may say to let it come in, or to pull it, or if the permanent tooth comes in near there, an orthodontist later might use braces to pull the stray (permanent) tooth into alignment where it belongs. Don't assume it's definitely one thing or another until you see a professional and she gets x-rays. (If she were older I'd say it's possibly the permanent tooth erupting behind the baby tooth -- that does happen and the kids have "shark teeth" where they can have two rows of teeth showing at once!)
See more than one dentist too if you can -- I've had friends get wildly different advice from different dentists on pull versus don't pull....But teeth come in and move around very slowly, so most dentists may say to wait and not be too worked up about it.
My daughter has the opposite problem; she is missing one permanent tooth as we know from her x-rays, but the dentist and orthodontist both agree we'll deal with the gap when she's much older, like later teens, because her head and mouth will still be growing for a very long time yet.