I had this happen to me as well. Assuming that everyone is ethical the other agent may have previously listed the property and the Trulia database has not recognised it. Make sure you are logged in to Trulia then go to the listing there should be a button I believe on the right had side...it will say something like claim this listing or co-list. Click on it and follow the instructions from there.
Now with that said the last time I did that I still ended up having to contact Trulia to get everything taken care of; it still had their hyperlink to their brokerage, their description of the property and their pictures as well so try to claim the listing and if that doesn't fix it contact Trulia and they should be able to help you out.