Yes, you probably could. Local person votes twice, 5 people commit fraud type stuff is incredibly hard to prevent and probably happens every election. As long as it requires X people to move X votes fraudulently, I would say we are going to be OK. Can you move a local election in a small town via this method? Probably, but our election system has defense in depth (recounts, election judges, polling, every-knows-everyone, voting receipts, etc. etc) so this isn't exactly easy.
It becomes a problem when 1-10 people can change the outcome of a district+ size voting block. "Hacking a voting machine" falls into this category. If 1-10 people can change a Red state to Blue via any method, that is what we should be concerned about. Again, defense in depth applies here. They would have to do many things perfectly AND get lucky for this to even be a possibility (no recounts, no voting machine inspection, no election judges intuition, etc. etc. etc.)
If my crazy uncle wants to vote twice, eh go ahead. Its difficult to do, takes a medium amount of effort and might work or might not.