pqReadData() error

Hi,

during execution of block of commands like

BEGIN
INSERT INTO...
INSERT INTO...
.....
INSERT INTO...
COMMIT

sometimes receive the error

pqReadData() -- backend closed the channel unexpectedly.
       This probably means the backend terminated abnormally
       before or while processing the request.

3 last lines in postmaster's log are:

ERROR:  Cannot insert a duplicate key into a unique index
ERROR:  Cannot insert a duplicate key into a unique index
FATAL 1:  my bits moved right off the end of the world!

Between BEGIN-COMMIT the program (written in C) makes 100 to 50000
INSERTs.
In the database I have two tables, say t1 and t2. Table t1 has an
unique index and two first lines from the error-messages above are
when the program tryis to make INSERT duplicate rows. But the error
pqReadData() I recieve when try to insert into the second table t2. It
hasn't the unique index.

My configurations is: postgresql 6.5.3 , RedHat 6.1 on Intel-based
server, RAM 256 Mb, swap 256 Mb, HDD 2x9Gb.

Best regards,
Alex