Dump of a mysql table on import replaced existing records
The mysqldump, by default, will drop the table. You should specified the --no-create-info option like this:

mysqldump -u... -p... --no-create-info --skip-extended-insert mydb t1 &gt; mydb_table.sql


That way, you only have inserts to deal with. Using --skip-extended-insert will insert one row at a time. This help deal with duplicate issues, but you will have import like this:

mysql -u...-p... --force mydb &lt; mydb_tables.sql


The --force option is for the sole purpose of continuing INSERTs in the event a duplicate key is encountered. In that instance, the offending INSERT's error is ignored and on to the next INSERT.