validation and data afterwards

[eluser]soupdragon[/eluser]
A quick question
I'm doing some validation in my controller works perfectly, so if all
okay go to my model and insert all into my db - also works.
they then get redirected to the next sucess page.

My question is er...are

1. How / where should i reset the _POST data
2. How to avoid when refresh pressed the data being inserted again? (as opposed to just checking in the db if already inserted)
3. Anyway to make the back button go elsewhere eg.over a header redirect

Not sure if that lot is clear ...when not shout

thanks