MySQL IN Statement Hang - mysql

Question

What I'm attempting to accomplish is return a SELECT statement of ALL duplicates in a table within given filters.
I'm attempting to run the following query but every time I run it my server locks up and it never completes the query. I have no idea what is causing this and some help would be greatly appreciated on either how to fix this or to accomplish my goal in another manner.
EDIT: I've added the EXPLAIN data as requested!
EDIT 2: I've added the CREATE statements as requested!
SELECT *
FROM red_flags
WHERE customer_number IN (SELECT customer_number
FROM red_flags
GROUP BY customer_number
HAVING COUNT(customer_number) > 1);
Execution plan:
1 PRIMARY leads ALL 80708 Using where
1 PRIMARY customers eq_ref PRIMARY PRIMARY 9 apcard_main.leads.customer_number 1
2 DEPENDENT SUBQUERY leads ALL 80708 Using where; Using temporary; Using filesort
2 DEPENDENT SUBQUERY customers eq_ref PRIMARY PRIMARY 9 apcard_main.leads.customer_number 1 Using index
DDL:
Table customers:
CREATE TABLE `customers` (
`customer_number` varchar(7) NOT NULL,
`dealer_id` varchar(32) DEFAULT NULL,
`first_name` varchar(64) DEFAULT NULL,
`middle_name` varchar(64) DEFAULT NULL,
`last_name` varchar(64) DEFAULT NULL,
`address_one` varchar(128) DEFAULT NULL,
`address_two` varchar(128) DEFAULT NULL,
`city` varchar(128) DEFAULT NULL,
`state` varchar(32) DEFAULT NULL,
`zip` char(5) DEFAULT NULL,
`fico` char(3) DEFAULT NULL,
`phone` varchar(13) DEFAULT NULL,
`mail_type` varchar(32) DEFAULT NULL,
`mail_date` date DEFAULT NULL,
`store` varchar(32) DEFAULT NULL,
PRIMARY KEY (`customer_number`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1$$
Table leads:
CREATE TABLE `leads` (
`lead_id` int(11) NOT NULL AUTO_INCREMENT,
`dealer_id` varchar(32) DEFAULT NULL,
`customer_number` varchar(7) DEFAULT NULL,
`date` date DEFAULT NULL,
`time` time DEFAULT NULL,
`source` varchar(32) DEFAULT NULL,
`source_type` varchar(32) DEFAULT NULL,
`home_phone` varchar(13) DEFAULT NULL,
`email` varchar(64) DEFAULT NULL,
`mail_type` varchar(8) DEFAULT NULL,
`store` varchar(16) DEFAULT NULL,
`first_name` varchar(64) DEFAULT NULL,
`last_name` varchar(64) DEFAULT NULL,
`middle_name` varchar(32) DEFAULT NULL,
`city` varchar(128) DEFAULT NULL,
`state` varchar(32) DEFAULT NULL,
`zip` char(5) DEFAULT NULL,
`fico` char(3) DEFAULT NULL,
`mail_date` date DEFAULT NULL,
`work_phone` varchar(13) DEFAULT NULL,
`cell_phone` varchar(13) DEFAULT NULL,
`address_one` varchar(128) DEFAULT NULL,
`address_two` varchar(128) DEFAULT NULL,
`comment` varchar(255) DEFAULT NULL,
`caller_id` varchar(128) DEFAULT NULL,
PRIMARY KEY (`lead_id`)
) ENGINE=MyISAM AUTO_INCREMENT=125587 DEFAULT CHARSET=latin1$$
View red_flags:
CREATE
ALGORITHM=UNDEFINED
DEFINER=`apcard`#`97.83.30.118`
SQL SECURITY DEFINER
VIEW `red_flags` AS
select
`leads`.`dealer_id` AS `dealer_id`,
`customers`.`phone` AS `phone`,
`leads`.`date` AS `date`,
`leads`.`time` AS `time`,
`leads`.`source` AS `source`,
`leads`.`customer_number` AS `customer_number`,
`leads`.`caller_id` AS `caller_id`,
`leads`.`mail_type` AS `mail_type`,
`leads`.`store` AS `store`,
`leads`.`last_name` AS `last_name`,
`leads`.`first_name` AS `first_name`,
`leads`.`city` AS `city`,
`leads`.`state` AS `state`,
`leads`.`zip` AS `zip`,
`leads`.`fico` AS `fico`,
`leads`.`mail_date` AS `mail_date`,
`leads`.`home_phone` AS `home_phone`,
`leads`.`email` AS `email`
from (`customers` join `leads`)
where ((`customers`.`customer_number` = `leads`.`customer_number`)
and (`leads`.`date` >= (now() - interval 30 day)))$$

Answer 1

Have you tried joining rather than using a subquery?
SELECT rf.*
FROM red_flags rf
inner join red_flags rf2
on (rf2.customer_number = rf.customer_number)
group by rf2.customer_number
having count(rf2.customer_number) >1

Answer 2

I figured it out, I had a poorly formed date filter that was causing it to break when querying with an IN for some reason.
leads.date >= now() - INTERVAL 30 DAY