I didn't know that capsuleers could never become Dusters. How does that make sense? What do the Space Marines do? How do the Dusters travel from planet to planet?
Because Dusters and capsuleers work entirely differently.
Capsuleer: Has a normal, human brain. This brain operates like brains normally do. When scanned, it is destroyed and a new one is grown inside the clone, which is once again a normal human brain. For the scan and transfer to be successful, the brain structure and neural mapping must be extremely similar between the two.
Duster: Does not have a normal, human brain. Their core consciousness is uploaded into a computer database. Their clones, meanwhile, have a sleeper implant attached to their brain, and this implant is linked to the database. Their organic brain, meanwhile, has been scrapped in favor of a hypercharged cerebellum to improve their capabilities in combat. There is nothing normal or human about the duster's brain, and ergo doing a transneural burn scan of it would just create a brain dead clone. When they die, there is no flash scan, the clone just dies and the central database receives a brief burst transmission of only variations experienced by the 'life' of that clone, which it then adds to the central file. The database then connects to the sleeper implant in the next clone. In essence, dusters are fundamentally computers, not humans, and their clones are quite literally just shells. Dusters are the true infomorphs, not capsuleers.
Ergo, it is not possible to be both at the same time.
It may be theoretically possible to convert between the two, however. At the very least, going from capsuleer to duster should be easily possible due to capsuleers being functionally still human. But this is basically a one-way thing as you, well, stop being human. Going from duster to capsuleer is more difficult, but as the duster's core consciousness is saved in a computer database, one could theoretically, if having all the specifications saved from when the duster was still human, grow a new brain with the neural map of that consciousness. As this wouldn't wipe the computer database, this would result in two versions of the same person: the duster version, saved on a computer database, and the capsuleer version. Due to CONCORD restrictions about multiple copies of the same person at the same time, doing this would be highly illegal unless the computer database was wiped afterwards.
All of this is described in EVE Source. Notably, it says the same thing I do about converting between the two, "The very act would immediately make the receiving vessel into a different person. It would, as the technologies presently operate, be an inevitable forking of personality into two distinct beings."
That is, of course, assuming that the process of uploading someone's neural map into the computer database does not fundamentally change its neural map. If it does, then there won't be any converting back to human.
For the purpose of this thread's topic, this is a good example of how the situation could be handled. One option would be to just quote the lore and say, "You can't do it, period. Stop." The other solution is to look at the lore, consider the in-character theoretical alternatives that are possible within the confines of the lore, and consider the IC situation as potentially one of those. Thus there is always the option for variations in local culture, experimental technologies, and so on.
The former, arguing about the situation OOC, takes the whole thing OOC, and thus is not preferable, while the latter, contemplating IC alternatives, leaves it IC. That is much superior in my opinion.