Before one import rows into already populated tables, one needs to
truncate or drop these tables to get rid of the old data. If not, the new
data will be appended to the existing tables. One must always DROP
existing Sequences before re-importing. If the sequences are not
dropped, they will generate numbers inconsistent with the rest of the
database. Note: It is also advisable to drop indexes before importing to
speed up the import process. Indexes can easily be recreated after the
data was successfully imported.