> I am sure it is something to do with locking or
> multiple insert context but I can find nothing in the firebird
> manual, in fact they imply there is no issue.

Firebird doesn't care how many inserts you do within a trigger,
certainly not a handful like you have here. This type of problem is
usually related recursion, like when you accidentally audit an
auditing table, or when your update trigger fires off an
insert/update/delete on a record that cascades through various
triggers and ends up updating the same record again.

Adam