> Habitable and inhabitable mean the same, don't they?

Yes, though "habitable" is the normal word and "inhabitable" would be a nonce construction derived from "inhabit".

> It's like[] inflammable and flammable.

Not really; those words mean that something can catch fire. That's all well and good for "flammable", but it's bizarre for "inflammable", since "inflame" has no surviving meaning related to fire. If you wanted to express that meaning, you'd have to say "set fire" or "ignite".