Some Social Security identification numbers are leading a double life as Internal Revenue Service employer identification numbers.

Apparently, the duplication has caused no harm so far. But it might do so if banks confused such numbers when sending customer information to the Internal Revenue Service, said Bruce Webster, a director at PricewaterhouseCoopers.

"Grandma could suddenly find herself investigated for money laundering because she had $10 million apparently come through her bank account," he said.

The problem seems to have arisen just recently. North Fork Bancorp on New York's Long Island says it spotted three cases of duplicate numbers in its own customer records, but Mr. Webster said he had never heard of such a thing before.

That such cases are uncommon suggests that the IRS and the Social Security Administration have cooperated to avoid duplicating each other's ID numbers, and that their coordination had somehow broken down, Mr. Webster and a North Fork executive said.

An IRS spokesman said the agency did a preliminary check in response to American Banker inquiries and found no other instances of duplicated ID numbers. More are possible, however, the spokesman said.

Though the IRS and Social Security numbers are each nine digits long, they are punctuated differently, he pointed out. Social Security numbers consist of three digits, a dash, two more digits, another dash, and then four more digits. The IRS' employer identification numbers are made up of two digits, a dash, then seven digits.

"The key thing is that the dashes are there," the IRS spokesman said. "Internally, we don't have any confusion, because of the dashes. It really depends on the software that is being used externally, in terms of how it recognizes numbers."

Mr. Webster said identical strings of numbers could confuse some banks' computers.

Many computer programs omit dashes when strings of digits are entered, he said.

Confusion is possible with bank forms that ask the customer to enter either a Social Security number or an employer ID number in the same data field. But Annie Early, a research director at GartnerGroup, pointed out that a saving feature is common in such cases: a linked field indicating which sort of number is being entered.

North Fork does distinguish between the two types of numbers, and caught the ones that matched before any confusion arose, the company executive said. The IRS confirmed that the numbers were correct and belonged to the customers in question, he said.

Anyway, said Mr. Webster of PricewaterhouseCoopers, such duplicated numbers would probably not affect basic transactions.

"I would trust that banks would do general work based on account numbers," he said, "and would use Social Security and tax numbers only for supplementary identification."

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