celery task insert duplicate data info mysql - mysql

Question

I am using celery to archive the async job in python, my code flow is as following:
celery task get some data from remote api
celery beat get the celery task result from celery backend which is redis and then insert the result into redis
but in step 2, before I insert result data into mysql, I check if the data is existed.although I do the check, the duplicate data still be inserted.
my code is as following:
def get_task_result(logger=None):
db = MySQLdb.connect(host=MYSQL_HOST, port=MYSQL_PORT, user=MYSQL_USER, passwd=MYSQL_PASSWD, db=MYSQL_DB, cursorclass=MySQLdb.cursors.DictCursor, use_unicode=True, charset='utf8')
cursor = db.cursor()
....
....
store_subdomain_result(db, cursor, asset_id, celery_task_result)
....
....
cursor.close()
db.close()
def store_subdomain_result(db, cursor, top_domain_id, celery_task_result, logger=None):
subdomain_list = celery_task_result.get('result').get('subdomain_list')
source = celery_task_result.get('result').get('source')
for domain in subdomain_list:
query_subdomain_sql = f'SELECT * FROM nw_asset WHERE domain="{domain}"'
cursor.execute(query_subdomain_sql)
sub_domain_result = cursor.fetchone()
if sub_domain_result:
asset_id = sub_domain_result.get('id')
existed_source = sub_domain_result.get('source')
if source not in existed_source:
new_source = f'{existed_source},{source}'
update_domain_sql = f'UPDATE nw_asset SET source="{new_source}" WHERE id={asset_id}'
cursor.execute(update_domain_sql)
db.commit()
else:
insert_subdomain_sql = f'INSERT INTO nw_asset(domain) values("{domain}")'
cursor.execute(insert_subdomain_sql)
db.commit()
I first select if the data is existed, if the data not existed, I will do the insert, the code is as following:
query_subdomain_sql = f'SELECT * FROM nw_asset WHERE domain="{domain}"'
cursor.execute(query_subdomain_sql)
sub_domain_result = cursor.fetchone()
I do this, but it still insert duplicate data, I can't understand this.
I google this question and some one says use insert ignore or relace into or unique index, but I want to know why the code not work as expectedly?
also, In my opinion, I think if there is some cache in mysql, when I do the select, the data not really into mysql it just in the flush, so the select will return none?