Here is what I do:

• Generate a unique key. (This is just be a large random #)
• Store the unique key in the submit form.
• Insert your data into the database with then Unique key being stored in a unique field.
• Now if the user submits twice (or 100 times) only one of the inserts will succeed.

This can fail if you are pick a random number that is already in the database (or you can check that) and it will fail if 2 user get the same unique key. You can't protect against this, but the odds are 1 in 4*10^9 if you use on a 32 bit key, but if you use a 64 bit key, then key collition is not be a problem at all.

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