I seem to be getting duplicates into my databases. I do not know why. Perhaps if someone hit the back button they enter a set of values twice. How could I stop this happening... to detect if the back button is been hit.

proper design of the database, including defining appropriate indexes should stop that. unlike a traditional computer system you have absolutely no way of controlling what a user does or doesn't. so you need to beef up your data model to ensure bad data cannot enter the system AND imnprove your fron t end to reduce the possibility of a user inadvertantly breaking your system