How many John Adamses were there in America in 1790? Too many to count. Adams was an extremely common family name in 18th century America, and John was the most common first name. In the early 20th century, there were two Winston ChurchilIs, a very well known (and now obscure) American writer and a minor (and now considered great) British politician and future prime minister. Sometimes letters of the minor ChurchilI surface as those of the prime minister through mistake. However, unscrupulous dealers on-line seem to be knowingly listing some letters of the "wrong" ChurchilI as merely letters of "Winston Churchill," without saying anything about the man, and people merely taking the name at face value are bidding on the near worthless items as if they were by the prime minister. They are stuck, because the letters were never described as being anything but what they are.
Also watch out for father/son pairs. Illustrated are examples of the signatures of George Ross, Senior, a signer of the Declaration of Independence, and of his son, George, Junior, an autographic nobody but a minor Pennsylvania official who signed many documents. Whether there’s something in the genes or not, many sons and grandsons have signed their names very much like their fathers or grandfathers did. In the Ross example, there is one clue- the father died in 1779 and the son didn’t sign much until 1780 and after. The son of Charles Dickens, also named Charles, had a signature that bore an uncanny resemblance to his father’s. President U.S. Grant and his grandson, U.S. Grant 3rd, are another striking example of similar signatures. The President died before the grandson was even born, so there is not likely to be confusion, but the resemblance is remarkable and illustrates the problems in this area.
So pay attention to place and date. Obviously a letter signed by Charles Dickens after 1870 is not written by the author (who died in 1869) and a letter signed by John Adams from Albany at a time THE John Adams was in Philadelphia also provides a clear answer.