That depends on the law applicable in your jurisdiction. In certain jurisdictions, it is illegal to open any package designated to another person. So if a package is accidentally delivered to your house, and it has another person's name clearly written on it, it would be illegal for you to open the package. You might not be obligated to return the package, but you also would not be able to use the contents without breaking the law. The law might also prevent you from destroying or discarding the package, in which case your options would be either to keep the package indefinitely, pass on the package to its intended recipient, return it to the sender, or break the law.