That code has a race condition that could explain the trouble you're having. Here's an example.

1. prog1 executes the select statement. max(id) is currently 3.
2. prog 2 executes the select statement. max(id) is still 3.
3. prog1 finishes out the code, incrementing id and then inserting all of the information for record with id 4.
4. prog2 finishes out the code, incrementing id and then inserting all of the information for record with id 4.

Depending on how your system is set up, the result could be that the second request is rejected (you don't do any error checking in your example, so you'd never know) or that both are inserted, and you only look at the first one your query returns (for example, in your sample select statement, you just ignore any additional rows that the query returns).

The solution is what chromatic suggested---make this action atomic somehow. Use an auto_increment variable, lock the table, or incorporate finding the ID into the SQL statement. As a kludgey alternative, require that the id column be distinct, then catch the error resulting from your insert statement, and get the new biggest id and try again.