Cool down emotions MM7 is extremely reluctant to give you the same atrifact twice. You will see that the more artifacts you have found the lower is your chance to get a new one. Check reports about duplicate artifacts in MM7 on this forum. You will find only a very few ones ... Therefore, it is really possible to get an artifact twice but it is extremely rare.
This puts a "limit" on the number of artifacts you can own: the limit is very close to the number of *different* artifacts in the game.

Note also, that most of artifacts are spoilt: they give you certain advantage (which is often dubious) but, perhaps as a "compensation", they also bring disadvantages (which may be serious). As a result, most of artifacts in MM7 are equal to useless trash, which isn't worth of carrying around.

If I can recommend something then I recommend giving up the artifact hunt. Waste of time and energy. Try to get the few artifacts in the game, which are really useful and concentrate on powerful "normal" weapons. A nice set of weapons of the Dragon plus an alternate set of weapons of Ice (for baddies immune to fire, nothing in the game, except medusas, which are immune to any magic, is immune to both fire and water) is much more useful than four full sets of artifacts and relics.