Attempting to add the same ship one after the other to the build queue results in resources being deducted, but the ship doesn't enter the queue. I've had some success waiting for the ship to be constructed then adding another one, although this is inconsistent; sometimes ships disappear along with the resource cost randomly. Behemoths are a special case where I cannot add another to any construction queue after building the first one. This may have had something to do with me deleting the normally un-removable military variant blueprint, but I haven't tested this.