C rank mercenary dies, activating artifact that returns him to when he first got isekai'd.
The world has people isekai from a magic world or murim martial art world.
The third isekai is a normal who can learn both martial arts and magic, but most of normals cant achieve much and end up really weak. Protagonist is a normal
Hes talentless but with no op powers aside from some knowledge of the future.
Tries to better his new life and runs into one of the strongest martial artists in the future.
This is Returning With Absolutely Nothing. It is licensed by Tapas.
In Aeria, a place where people from various worlds are summoned, the weak are destined to die. Lee Sungmin, a talentless mercenary, was no exception. Fortunately for him, a mysterious item he finds before his untimely death sends him back to the past. Now a 13-year-old with a second chance at life, Sungmin begins his desperate journey to become Aeria's strongest with no money or talent. The question is... will effort alone be enough to make a hero of a boy with absolutely nothing?
The story opens with the main character being summoned to another world; seemingly for the first time, but when someone tries to poison him he turns the tables on the assailant as he already knows the trick. It's revealed that he went back in time 13 years to when he was first summoned. He is explicitly referred to as a C-rank, and a "classless"
He's not quite talentless; besides knowing future events, it's stated that the classless have higher growth. They just usually don't live long enough to do anything with it, as they're initially very weak.
In the second chapter he runs into a small child as he's first summoned;
and realizes that this person is a master martial artist that was summoned at about the same time as him in his first life.
Found by searching mangaupdates for the tags Murim and Isekai.
