I often use "atheist" to mean someone who disbelieves in supernatural phenomena when I really shouldn't. Is there a better term for this?
Naturalist (as opposed to supernaturalist), materialist or physicalist are possible alternatives that might be of use to you.
Why aren't your replicas you? If you take my example, where the replicas have a chemical structure where each molecule is arranged with identical position and momentum to yours (down to the limitations imposed by quantum uncertainty), what scientific test could possibly distinguish it from you?
An interesting thought experiment to highlight your point would be teleportation. If teleportation worked by essentially destroying the body so that any observer would conclude that the person was very much dead, but if it also creating an exact duplicate in an alternative location, did the person actually die? I would say no, they haven't; but it is an interesting philosophical question and I wouldn't dream of concluding that my intuition on the subject was conclusive.
An interesting variation of this theme can be found in the afterlife proposed by Philip Jose Farmer in his Riverworld saga. In this scenario aliens have used supertechnology to record all the relevant variables that go into making a person a person up to just before the moment of death. At a certain point in time they then recreate all these persons at once on the banks of a designed river that runs the length of the planet. So there are 12th Century people from Languedoc interacting with 20th Century Americans all with the subjective impression that they had just died and woken in a strange afterlife.
Does this count as far as afterlifes go? Sure. Can a materialist believe it is philosophically possible to therefore have an afterlife? Absolutely. They might think it is highly unlikely or technically impossible - but they could easily concede that it would be, for all intents and purposes, an afterlife.
Though there would still be the question of 'am I who I think I was?'. Which sounds nonsensical, but it is compelling to some people and it isn't inherently trivial as far as I can see.