Jim Mundy owned a spread twenty miles south of Bonita, near the border, and he was always suspected of crimes like rustling. Mundy and his men used their reputations to intimidate the people of Bonita, and he had designs on stealing the miners' payroll from the town's bank. (All of this was depicted in the 'Maverick' episode "Seed of Deception".)
And he must have been a master of disguise, fooling even Wyatt Earp!
When he heard that Wyatt Earp and Doc Holliday were in town (although they were really Bret and Bart Maverick), Mundy mentioned that he was first cousins to the Clantons.
This would splain why Jim Mundy was also one of those Doctor John Holliday impersonators found throughout the Old West.
Doc Holliday is one of those historical figures who was plagued by copycats, people with FIS (Famous Individual Syndrome or Famous Impersonator Syndrome) who could only find attention for themselves by pretending to be somebody else. This was an epidemic situation in the Wild, Wild West, where reputation was something to be desired. Even Charles Dickens fell victim to it - he never got past the Mississippi on his American speaking tour, and yet there he supposedly was in Virginia City with the Cartwrights, as seen on 'Bonanza'!
By the end of "Seed Of Deception," yet another Doc Holliday impersonator had shown up in Bonita, one who had dealt with the Mavericks at least one other time in the past and who also had been spotted in 'Tombstone Territory'.
There was a long stretch of time when Doc Holliday wasn't around in Dodge City, and Jim Mundy took advantage of that to spy on Wyatt Earp for his kinfolk the Clantons. They must have known the lawman was planning on leaving Dodge to go clean up Tombstone and the Clantons probably wanted to know who they would be up against if they wanted to keep to their lawless ways. So they sent Jim Mundy in to impersonate Doc while he was gone in order to gather "intel" on Earp.
And apparently Wyatt Earp never noticed; either that, or he was playing the long game in order to see what this impostor's ultimate plan was.
Eventually the real Doc Holliday returned, this time to Tombstone, and Jim Mundy snuck back to his ranch near Bonita, California. There he finally met his match in the Maverick brothers and was caught red-handed trying to rob the town's bank.
Jim Mundy was probably an old man by the time he got out of prison.....