Checking For Unique Constraints During A Batch Update

hello .
i have a grid for a table that gets updated with recordset.updatebatch
for a multi-user application.

the problem is that some of the table fields have to be unique

now imagine the next situation.

user A opens the form
user B opens the form
user A writes '1000' in the unique field
user A saves the recordset with updatebatch

user B writes '1000' in the unique field
user B saves the recordset with updatebatch

now there will be two records with the field '1000' !

how can i avoid this ?
i cannot check for unique during the update event of the grid
because it should check during the time it is saving and not
when it is just entering data without having updated the recordset

thanks !