Nervala IV is an inhospitable planet whose atmosphere is wrapped in a highly unstable distortion field, allowing transport only once every eight years. The planets surface is as lethal as the atmosphere is unstable, the research facility needed a shield to protect the facilities inhabitabnts from the lethal radiation outside.
During an attempt to evacuate a Starfleet research team during 2361, Lieutenant William T. Riker, who was on temporary assignment aboard the USS Potempkin, was nearly trapped on the planet as the transport window began closing in on him, in order to get him out safely, a second confinement beam was used to strengthen his signal.
When the transporter chief aboard the Potempkin got Riker to safety, the second beam was cut off, but rather than breaking down to nothing, the second beam, which apparently bounced off of the atmosphere's distortion field, was sent back to the surface, where an exact duplicate of Riker materialised back inside the facility where he lived for eight years until he was rescued in 2369.
In that year, Commander Riker, then serving as first officer under Captain Jean-Luc Picard on board the USS Enterprise-D, returned to the planet with an away team to collect the research that was left behind eight years ago.