pgsql-function called twice in the same second -> Double Insert -> Error

Hi there,

I am administering an online (web) learning portal (OpenACS + Postgres)
where students can solve multiple choice exercises online. Every result
of a exercise usage is saved to the database. Students can see which
exercises they have solved successfully, and other stats.

The whole exercise and testing module works quite good since weeks, but
from time to time I noticed error messages. "cannot insert duplicate key
into unique index..."

Now I think have found the problem: There is a db-function that saves
the result of every exercise usage, and this function is sometimes
called twice in the exact same second (when the server is extremely
busy). Probably the user clicked two times on the form submit button in
the browser.

The exercise usage process consists of 2 steps: When the student starts
the exercise the corresponding entry is "locked". (the id of the
exercise is inserted into a locking table) Every exercise usage
generates a usage_id that is recorded in the locking table, so the
exercise can not be deleted while used. When an answer to the exercise
is received, the exercise usage data is inserted into another table and
the locking entry is removed.

So the function first tries to retrieve data from the locking table,
saves (inserts) that data in another table and deletes the entry from
the locking table. If this function is called twice at the same time,
both runs get the data from the locking table and both try to do the
insert. in the first run, the commit succeeds, but in the second run an
error is generated and the function fails.

Is this something I can circumvent on the Postgres (DB) Level or do I
have to deal with that at the application level (Aolserver - TCL) ?

Many TIA!

peter

PS: The db-function I mentioned is attached.