Mysql 5.5.38 ERROR 1062 (23000): Duplicate entry ” for key ‘anykey’

I dropped my UNIQUE KEY index 'myindex' on a table
and I wanted to change the UNIQUE KEY to something else,
so after dropping it I did:

alter table mytable add unique key anykey (add1, city, state);

I now get the following message on a key that does not exist
(it doesnt seem to matter what key I change 'anykey' to, I always
get the same error:

ERROR 1062 (23000): Duplicate entry '' for key 'anykey'

I dumped the entire DB and reloaded it, but still cannot seem to set
a unique key on 'mytable'.

Anyone know what I should check or how to get around this issue?
I WANT a unique key on this table.

The tables important parts:

PRIMARY KEY (`id`)
ENGINE=MyISAM AUTO_INCREMENT=3433 DEFAULT CHARSET=latin1

Thank you in advance for any help!

EDIT: here's show create table

| mytable | CREATE TABLE `mytable` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(50) NOT NULL,
`name_aka` varchar(100) DEFAULT NULL,
`in_add1` varchar(50) NOT NULL,
`in_add2` varchar(50) NOT NULL,
`in_city` varchar(50) NOT NULL,
`in_state` varchar(2) NOT NULL,
`in_zip` varchar(5) NOT NULL,
`in_zip4` varchar(4) NOT NULL,
`ship_add1` varchar(50) NOT NULL,
`ship_add2` varchar(50) NOT NULL,
`ship_city` varchar(50) NOT NULL,
`ship_state` varchar(2) NOT NULL,
`ship_zip` varchar(5) NOT NULL,
`ship_zip4` varchar(4) NOT NULL,
`phone` varchar(14) NOT NULL,
`phoneExt` varchar(6) NOT NULL,
`phone2` varchar(14) NOT NULL,
`phone2Ext` varchar(6) NOT NULL,
`fac_fax` varchar(6) NOT NULL,
`gender` varchar(7) NOT NULL,
`newsletter_ok` enum('Y','N') DEFAULT 'Y',
`notes` varchar(300) NOT NULL,
`fac_type` varchar(14) NOT NULL,
`fac_contact_email` varchar(50) NOT NULL,
`ind_eng_bks_shipped` varchar(6) NOT NULL,
`ind_span_bks_shipped` varchar(6) NOT NULL,
`ind_eng_s_bks_shipped` varchar(6) NOT NULL,
`ind_span_s_bks_shipped` varchar(6) NOT NULL,
`indiv_other3_bks_shipped` varchar(6) DEFAULT '0',
`cases_eng_bks_shipped` varchar(6) DEFAULT '0',
`cases_span_bks_shipped` varchar(6) DEFAULT '0',
`cases_other1_bks_shipped` varchar(6) DEFAULT '0',
`cases_other2_bks_shipped` varchar(6) DEFAULT '0',
`cases_other3_bks_shipped` varchar(6) DEFAULT '0',
`last_update_by` varchar(20) NOT NULL,
`last_update_date` datetime NOT NULL,
`rev` varchar(9) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=3435 DEFAULT CHARSET=latin1 |

I removed the —- stuff obviously.