Any property in the world can have an offer written on it. However, the question that should be asked is if the seller will be receptive.
There are three main statuses involving "For Sale" properties in the Real Estate world.
Will the seller be receptive when you have your agent write a offer on a property that has already had an accepted offer written? Receptive, they might be. But if they kick the current buyer out of escrow, they could face legal consequences.
Usually, we will keep an offer written on one of our listings in Back Up position if we get an offer after a Status of Pending has been entered into the Multiple Listing Service.
Contingency sales. I suppose that the "real estate question asker" meant a property that is for sale where the seller has to complete some type of action before or during the listing time frame, or there after. Such cases can include the seller needing to find their home of choice, needing to close another escrow on a different property, amongst others.
If a current buyer is in escrow and has contingencies of their own, such as needing to sell their own home before being able to buy the "sellers", you might be able to take the "pole position" by writing an offer.
In California, at the offering time, the sellers real estate agent would write up and have the "current contingent buyer" served with a 72 1st right of refusal. Basically meaning the current contingent buyer has 72 hours to waive their current contingencies or the seller will kick them out of escrow and sign up the new "non contingent" buyer.