How can i avoid inserting duplicates in my datalist ?

Can you expand the question?

Depending on the storage method, the system actually prevents you from saving identical records, securing it with a unique ID (for example through multirow).

However, if, for example, you want to prevent a field from taking the same value in two or more records, you would have to develop a validation of the list through beanshell.

i 'am using a form, and from that form i make a datalist. Do i have to use beanshell ?If yes can you link me some code example to make that ? 
Regards