LONGMONT, Colo. -- Your Social Security number is supposed to be your number and your number only, but one Longmont woman discovered that someone else had hers.

Valorie Sundby said she discovered her headache of a problem last year while trying to file her taxes online.

"They said someone has been filing under this number for years," said Sundby.

For 23 years, Sundby and another woman shared a Social Security number and their lives became commingled, making the two women, in the eyes of the government, one person.

"The headaches, the stress. I had to quit my job because I was so stressed out," Sundby said.

For days, Sundby flipped through documents trying to correct a mistake that wasn't her fault. Government officials say what happened was that in 1980 a California woman who had lost her Social Security card applied for a duplicate one but she wrote down the wrong Social Security number -- Sundby's number.

"This one slipped through the cracks and we issued it that way when we should not have," said Delia Lasant with the Social Security Administration.

It's a mistake the administration says is rare now because in this modern age, everything runs through computers. Also, they say if a person looks at his or her local earnings -- information that is mailed every year -- they can catch the problem before it becomes the huge headache that consumed Sundby.

"If something doesn't look right, you have too much money, not enough or none, you need to contact us immediately," said Lasant.

After nine months of paperwork, the SSA fixed Sundby's earning records. Life can move forward but Sundby says she will always be looking behind.

"I am always going to have to be looking over my shoulders," she said.