Most homes do close on time but things can always happen. If you are working with a Buyer's agent then it is the buyer's agent's responsiblity to keep tabs on the property for you. If this is the house you want and you lost out the first time, then I would go ahead and put in a back up offer. In my opinion it is not a waste of time. If this is not the house, then I would just keep an eye on it and keep looking.
But keep in mind, realtors and sellers in most cases will do what they can to notify all interested parties of when a property comes back on the market but it cannot be guarantee. Someone is always going to be upset and after 10 yr in the business it always seems to be "my realtors" fault. SO again, if this the house you want, I would go ahead and put in a back up offer. Good luck and please let me know if you do get the house. I love to hear success stories....smile.