The incident, which apparently occurred on Monday afternoon, came just weeks after the civil rights and privacy group was fined $10,000 by the New York Attorney General's Office for exposing the personal details, including names and phone numbers, of approximately 90 people who made on-line purchases through an ACLU web site.
The group had agreed to implement changes to prevent similar errors from happening in the future.
Monday's e-mail was reportedly sent to 900 people, whose details ACLU collected via the phone or the web. According to The Washington Post, the ACLU admitted that, when the mistake was discovered, a recall e-mail that repeated the mistake was sent.
The Attorney General's Office has reportedly said it will review the new case.
Two years ago, ACLU had requested the US Federal Trade Commission to investigate an incident where an e-mail error by pharmaceutical giant Eli Lilly exposed sensitive personal information collected from consumers through its Prozac.com web site.
The ACLU petition resulted to the company being levied a $160,000 fine divided among the states of New York, Massachusetts, Connecticut, Idaho, Iowa, New Jersey, Vermont and California.
Eli Lilly was also ordered to review its privacy practices, and to undergo annual, independent compliance reviews over the next 5 years and report the findings to the states.