In the case of Star Trek IV in which Gillian Taylor jumped into Kirk's transporter beam, this forcefield maybe even prevent objects from fusing together.
If it detects that two objects are significantly different enough (so DNA-wise in the case of living things), or different enough on a molecular level (two identical to the naked eye boxes).
If two identical twins are beamed at the same time, might they be accidentally combined because the transporter mistook them for one person? Or does the transporter detect epigenetic differences, too?