Duplicate values present in primary key column in postgresql

I have a table in which one of the columns is primary key and it is of data type citext.

Many threads will update the table, they are properly synchronized. Those threads write the data in files(separate file for each thread) and then dump the data into a temp table which also has same column as primary key and then update the main table using

update tabl1.col1 = temp1.col1, ...
where tabl1.pkcol = temp1.pkcol

and then insert new data using

insert into tabl1 (col1, col2, ...)
select col1, col2, ...
from temp1 where not exists (select * from temp1 where tabl1.col1 = temp1.col1)

Now I have many duplicates in the primary key column.

Environment Details:

OS: Win server 2008 r2 64 bit

Postgres version 9.2.4

Used by a java(java 1.5) application running in the same machine.

I checked for any abnormal stoppages of java/ postgres which had not happened.

After dropping and re creating the table it is working normally under same conditions.

Under what scenario such duplicate entries would be inserted? Why the DB didnt throw any error while duplicate value was being inserted?

Any help or pointers would be appreciated.