I've seen the following idea implemented: You can cast your vote as often as you want with only the last one counting. Sure, in practice there are a lot of things to consider to make it foolproof and it might be impossible to do securely - I don't know. But it's worth considering.
It would help against forced votes. But it's useless if some lazy person wants to sell his vote. With regular voting, there's a minor barrier in place. The incentivized voter has to go to the polling station and bring back some kind of proof. With online voting, someone could just take his ID (or e-signature device) and vote himself. The voter selling his vote may even don't know whom he "voted" for. Furthermore, someone might buy those signature devices off homeless/addicts/etc and build a massive voting farm.
