I have a hypothesis about it Perhaps, if the game rolls an artifact and if it sees that you already have the artifact, then the roll is cancelled and a new roll is made. Perhaps, the new roll is valid no matter what it contains. Consequences: (1) The more artifacts you have the less often you get a new one (the more rolls get cancelled). Correct. (2) You usually get only one copy of a specified artifact (there is a really small chance that the repeated roll will contain the second copy of the artifact again). Correct. (3) Nevertheless, it is possible to have more than one copy of a specified artifact. Correct.