How can I  manage records with the same values at a specific field ?

I have written a module that has a field named as, for example, “asset-num” and this field Is initialized by a specific user.

Therefore I don’t want to define its type as “sequence” ( I don’t want the system to initialize It automatically neither  based on sequential numbers)

Therefore, how can I manage the situation when two records( a form) with equal value of  “asset-num” field are registered , the system yields an error ?

(openerp 6.1)