The so-called doppelganger's name is Severin Beckwith ... he was hiking the Appalachian Trail with a pal and staying at a hotel called the Fontana Village Resort & Marina in North Carolina recently -- and U.S. Marshals gave him a rude awakening.

Beckwith tells the New Yorker he heard a knock at their hotel room door ... and before he knew it, a gang of federal agents with riot gear and shields were busting in, guns raised and pointed at his face -- proceeding to pin him down and check for signs he was Laundrie.

Apparently, one of the agents who was checking him out said he had a notch in the upper part of his inner ear -- just like Laundrie -- but luckily, Beckwith was eventually cleared.

After providing an ID and showing he didn't have Brian's tattoos, Beckwith was cut loose ... while also being warned he should shave his beard for the foreseeable future. Beckwith's rocking a shaved look (like Laundrie, who's bald). Witnesses in that area claimed to have seen Laundrie lately.

As for who ratted out Beckwith ... he says he suspects it was one of the hotel employees, who apparently fired off a photo too, which the agents showed him during the ordeal.