>>
>> && put the flock() before the locate
>> && or
>> go (recno()) && get latest data from disk
>
>OK, I moved FLOCK() before locate.
>Unfortunately, I have no idea how this can helo in my case: there is always key record in keys table.
>No records are added or deleted from keys table.
>duplicate keys occur when many users are using the same keys table record.

I'm not 100% sure that flock() rereads the record from disk.
It does not harm to put the flock() before the locate
Maybe it will help, maybe not