A temporal duplicate was a duplicate of an object after traveling through time.
Such was the case of the DeLorean time machine, in which during the Hill Valley Thunderstorm when it was struck by lightning, sending Emmett Brown back seventy years to 1885, a temporal duplicate was sent forward in time seventy years to 2025.
This version was later recovered by Doc on one of his trips to the future, and he added new features to the existing ones (hover conversion, Mr. Fusion etc.) — such as spatial displacement and an automatic retrieval feature. This version was "piloted" by Einstein, and then automatically returned to 1986 in front of Doc's garage to the amazement of Marty McFly.
The only known temporal duplicate is the one caused by the lightning bolt accident described above, and as neither of the time travellers (Marty, the Brown Claytons and Jennifer) knows exactly how the unstable lightning bolt affected the time circuits and DeLorean, the chance of ever repeating the phenomenon is slim.