URGENT: Same value restriction on a text field

Hi,

I want to apply a restriction on a text field such that once a particular value is saved in a record; the same value can't be used in the same field for a new record. How can I achieve this? I tried using PRE function both in field validation & workflow but that didn't help.

Can workflow / field validation on a text field help checking the field value duplication in comparison to previous records?

Example:

Object - Account

ABC ( Text field) - ABC123 (Value)

Now while creating a NEW record, if I try saving the same above value in the respective text field; it shouldn't allow the same & rather display an error message i.e. This value is already used in one of the existing records).