Can I make an offer on a house that already has a contract?
Sometimes. In some cases, sellers will be open to accepting back up offers - if the home is listed for sale through an agent, or if you have a buyer's agent working for you, the agent can usually determine this.
This is often done especially if the financing of the original buyer is in question, or if there is something about the property that makes it a high risk for a cancelled contract (such as home inspection issues, title oddities, repairs, etc.)
However, if the home is under contract, your offer (even if it is for more) cannot usually be automatically accepted - the existing contract must be allowed to play out according to the terms determined by those parties. If the first buyers fail to deliver, THEN your offer is considered.
Approach your agent, the list agent, or the seller if you are interested in placing a back-up offer. You will quickly determine whether or not this is an option.