Robert Merton found 261 examples in 1961, and observed that the phenomenon of multiple discovery was itself a multiple discovery, having been described over and over again since at least the early 19th century. This required specifying what exactly we mean by “multiple.” I ended up using three different categories of multiple invention. The second category of multiple invention I used was cases where there were multiple serious efforts to actually create a given invention. Overall, I found that within this sample of historic inventions, multiple invention was extremely common. We can also look at whether the frequency of multiple invention varies by the type of invention.