“Do this only once each turn” refers only to the ability it is attached to, not other instances of the same ability. In other words, if you control multiple copies of Riveteers Ascendancy, you will be able to do this once for each of them.
Riveteers Ascendancy
If multiple instances of the ability are on the stack, you will be able to return a creature to the battlefield for only one of those instances. Once you do that, other instances will do nothing as they resolve.
Once you have chosen to return a creature to the battlefield, further instances of sacrificing creatures the same turn will not cause the ability to trigger.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Svetlin Velinov
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
