Actually, the Drop Mechanics have been outlined in the Dev Blog. The mechanics are based off of the previously announced system of how Crates/Boxes determine outcomes, which was released a while back as the method of labeling contents and drop rates was unveiled.
So you can no longer "Roll a ship you already have". That no longer exists.
A) Containers now dynamically roll for their outcome when they are opened. Previously, their contents were decided at Purchase, which allowed a player to have "repeat" ships as multiple containers could have rolled the same outcome.
B) When a ship is owned by the player, it is disabled from the possible loot table. The remaining ships in a group that are not owned are evenly weighted.
Example: A "ship group" has 20 ships in it, but the player owns 5 of them. If the player rolls that "ship group", then each of the 15 non-owned ships has a 1/15 chance of being picked.
I've heard some folks surprised that you could get Steel if you have ALL the ships available via the Santa Crate. As I can't imagine there are too many folks that literally have ALL of them, I don't expect this to be an oft traveled road.
Even then, the "steel" awarded would only happen if the container rolled a ship group after all the ship option were exhausted. It is very unlikely this will be of great significance.