A person goes to the voting center and gets a ballot That person goes to the booth and marks the ballot That person skips entering their ballot and goes out Shows the ballot to the coercer, verifying the vote The coercer gives the ballot to the next person The next person gets another ballot and has the previous one hidden. That person introduces the new ballot, hides the old one and goes out. And the chain goes on.
Apparently it's a common way to coerce votes in low income urban areas and rural areas. You only need distraction or complicity from a person from the voting table and it's hard to detect.
Another common issue is vote stuffing.
On the philosophical part, it is in the end a human problem, but with technology at least you should reduce the possibility of cheating