To prevent somebody to enter values twice you can apply a conditional
format to show a red background if the value already exists, for
example.

If your project is of some significant size - let's say like setting
up bank account numbers or ISBN's - then you might consider setting up
check digits: http://en.wikipedia.org/wiki/Check_digit

[20% of all unchecked human data input is wrong]

Regards,
Bernd