Turns out there is a physical trait of identical twins that isn’t identical. Fingerprints, here is why:
Because fingerprints are influenced by environmental factors during intrauterine life.
…
Although very similar, the fingerprints of identical twins are nevertheless unique because they are not completely determined by genetics: they are an example of phenotype, which refers to the physical characteristics of an individual that are determined by the interaction of genes and the environment.
The New York Times covered this back in 2004:
While a fetus is developing, the ridges along these patterns are influenced by a number of factors, including bone growth, pressures within the womb and contact with amniotic fluid.
For all the talk about how no two people have identical fingerprints, I hadn’t thought about the specific case of identical twins. Are there any other physical tests that you can do to tell the difference between identical twins?