In my opinion, there are some cases of dead people voting along with people who are registered in two or more states. However, I suspect those numbers are rather small compared to the total number of votes cast. What I believe to be a much more significant problem is illegal immigrants voting.
There are currently 12 States and Washington, DC that allow driver's licenses to be issued to people in the country Illegally. These licenses can in turn be used as identification for people to vote in a number of states. Add-in fraudulent documents such as counterfeit Social Security cards, fake “green cards,” and phony birth certificates and you can see that a determined individual would have no problem voting.
According to the Pew Research Center, there were 11.1 million unauthorized immigrants in the U.S. in 2014. California has the largest number of illegal immigrants with an estimated 2.4 million unauthorized immigrants making up about 6.3 percent of the state's total population.
Common sense suggests that some illegals voted in the national election. The question is... how many?