Hi,

 In your model add this function,

from odoo.exceptions import  ValidationError

@api.constrains('project_reference')

    def _check_duplicate(self):

        for each in self:

            if each.project_reference and each.project_reference != 'PR0000':

                domain = [('project_reference', '=', each.project_reference)]

                all_forms = self.search_count(domain)

                if all_forms:

                    raise ValidationError(_('The project reference already exists!'))

Hope it helps,

Thanks