I just had a new potential explanation for an awkward issue...
About 25 years before Voyager, the Paris family was having some difficulties with fertility. So, they went to a clinic for some assistance. The doctor on duty was long on narcissism and short on ethics, so he used illegal triphasic cloning techniques to create an embryonic duplicate of himself, which he placed in Mrs. Paris's womb. He had done a similar procedure with Mrs. Lacarno a few months before, and maybe one or two others as well (just in case Robbie MacNeil played an extra or two somewhere). He was careful to choose only couples with the right features, and hair/skin/eye color, so no one would suspect. Eventually, the doc realized that he would get caught if he kept on, and desisted. No investigation ever occured. Which explains why Tom Paris and Nick Lacarno looked and acted so similar: despite different upbringings, they were genetically identical.