Bartholomew is the clearest case in the twelve of a name that may not belong to a separate person, and the identification is old enough to have hardened into a fact.
He appears in the three synoptic Gospels and in Acts only in the lists of the twelve, always as a bare name, always paired with or adjacent to Philip. Nothing else is recorded of him: no saying, no scene, no action. The name itself is a patronymic rather than a given name, Bar-Tolmai, son of Tolmai, which means the lists are giving us a man identified by his father, and his own name is missing.
John never mentions a Bartholomew. Instead John has Nathanael, brought to Jesus by Philip, who asks whether anything good can come out of Nazareth and is told that Jesus saw him under the fig tree. Nathanael appears again in the closing chapter among a group of disciples fishing, in company otherwise made up of named members of the twelve.
The standard harmonisation, current since at least the ninth century in the West and widely accepted, is that Nathanael is Bartholomew: the synoptics give the patronymic, John gives the personal name, and Philip introduces him in both. It is a reasonable inference. It is not stated anywhere in the texts, no first-millennium writer of the first few centuries makes it, and it rests entirely on the Philip association plus the absence of either name from the other tradition. Some scholars accept it, others treat Nathanael as an unrelated figure or as a representative rather than historical character. Filing them as one person is a decision, and it should be visible as one.