Look at http://www.craigberntson.com/Articles/kb006.htm

>The following code was written for VFP5 a few years ago. Either the customer never noticed there was a problem, or it's just started acting up, but when I was testing it last week, it kept ending up with duplicate record ID.
>...
> set deleted on
> set reprocess to 30 seconds
> use sel_control in 0 exclusive && this is a single record table
> current_request_id=sel_control.next_request
> replace sel_control.next_request with current_request_id+1
> use
> close all
> set exclusive off
>...
>later on in the code
>* Create the main request row for the request_id
> m.request_id = m.current_request_id
> m.fieldxxx ....
> m.fieldyyy = ... etc.
>...
> insert into sel_request from memvar
>At this point it fails sometimes, and will not do the insert, because request_id which is supposed to be a unique field in this table ahs already been used. However, the first piece of code should have incremented it by one, each time.
>
>This was failing, when only one user was on the system, so it's not from too many multiple users.