I had a picture of a flower with an insect on it. Had to post it twice, once in plants and once in inverts. A duplicate entry system would have saved some typing and writing down the map reference to copy into the second entry.
I agree that could be a useful feature for those occasions when there are two things in one picture requiring one ID (I've done that myself a few times).
However once you have created the site for the first time simply typing its name (rather than the map reference) will be enough to get iSpot to complete it.