Inconsistency between mysql dump file and binlog

Question

I'm trying to do a point-in-time-recovery of our production database, but when reading the binary logs after restoring the dump, I get ERROR 1062 (23000) at line x in file: 'binlogs_file.sql': Duplicate entry 'y' for key 'PRIMARY'. I've double checked, and it seems insert statements in the binary logs already exists in the dump file, and consequently in the newly restored test database.

This is my mysqldump statement that runs every morning on our production server:

echo "SET autocommit=0;" >> backup_file.sql
echo "SET unique_checks=0;" >> backup_file.sql
echo "SET foreign_key_checks=0;" >> backup_file.sql
mysqldump --flush-logs --quick --single-transaction --master-data=2 --force --  routines  <databese_name> >> backup_file.sql`
echo "COMMIT;" >> backup_file.sql
echo "SET unique_checks=1;" >> backup_file.sql
echo "SET foreign_key_checks=1;" >> backup_file.sql

I copy the latest dump to the test server (which is a snapshot of our production server, only three weeks old). After restoring the test db, I retrieve what binlog file to start from, with this command:

cat backup_file.sql | grep 'CHANGE MASTER TO MASTER_LOG_FILE'

and this is returned:

CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.006502', MASTER_LOG_POS=154;

I copy the mysql-bin.006502 file, as well as all the succeeding binlog files from production server, and create one .sql file out of them all (note that the position is superfluous, as --flush-logs is part of the mysqldump statement)

mysqlbinlog mysql-bin.006502 > binlogs_file.sql
mysqlbinlog mysql-bin.006503 >> binlogs_file.sql
mysqlbinlog mysql-bin.006504 >> binlogs_file.sql 

Next step is to run the binlogs_file.sql against the db.

mysql -u <db_user> -p -e "source binlogs_file.sql"

Then the error occur:

ERROR 1062 (23000) at line x in file: 'binlogs_file.sql': Duplicate entry 'y' for key 'PRIMARY'

We're running MySql 5.7.19 on both production server, and test server (both Debian 8.10). These are the binlog related variables on the production server:

binlog_format                   = mixed
binlog_group_commit_sync_delay   =   0                                                                                                                              
binlog_group_commit_sync_no_delay_count = 0 

What am I doing wrong? Why the inconsistency?