Insurers say the federal health-care marketplace is generating flawed data that is straining their ability to handle even the trickle of enrollees who have gotten through so far, in a sign that technological problems extend further than the website traffic and software issues already identified.

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Emerging errors include duplicate enrollments, spouses reported as children, missing data fields and suspect eligibility determinations, say executives at more than a dozen health plans. Blue Cross & Blue Shield of Nebraska said it had to hire temporary workers to contact new customers directly to resolve inaccuracies in submissions. Medical Mutual of Ohio said one customer had successfully signed up for three of its plans.

The flaws could do lasting damage to the law if customers are deterred from signing up or mistakenly believe they have obtained coverage.

“The longer this takes to resolve . . . the harder it will be to get people to [come back and] sign up,” said Aetna Inc. US:AET Chief Executive Mark Bertolini. “It’s not off to a great start,” he said, though he believes the marketplaces are “here to stay.”

An expanded version of this report appears at WSJ.com.