Having struggled to fathom Medicare's new drug coverage, tens of thousands of beneficiaries are perplexed to find themselves actively enrolled in two prescription drug plans at the same time.

Shirley D. Beer, who lives in a mobile home here in western Pennsylvania, illustrates the predicament. Mrs. Beer was assigned to one insurance plan by the federal government, then exercised her right to choose another plan and now seems deeply embedded in both.

The first plan still carries her on its rolls and pays for some of her prescriptions, even though she declared four months ago that she wanted to drop out and had joined the second plan, which covers most of the 12 medications she takes for heart problems, high blood pressure and other chronic conditions.

Many Medicare beneficiaries like Mrs. Beer are entangled in two live plans - two "hot plans," in the lingo of pharmacists. The situation leaves patients at risk of being charged two premiums or incorrect co-payments.

The Bush administration acknowledged the problem in a recent memorandum to insurers.

"Numerous beneficiaries have switched plans," the memorandum said. "Our processing systems have not always sent the enrollment and disenrollment information to the appropriate plans." As a result, it said, "many (possibly all) of the beneficiaries who switched plans are active on enrollment files at multiple plans." 

The situation illustrates the "computer glitches" that officials say have bedeviled the program since enrollment began in November.

The Medicare agency said it was working with insurers to resolve the "enrollment discrepancies." But it is proceeding with caution because, it said, a sudden, unexpected mass disenrollment could provoke a flood of inquiries and complaints to insurers and the government from puzzled beneficiaries.

Mrs. Beer, a 78-year-old widow who lives here outside Pittsburgh on a government check of $603 a month, said that her first plan, Gateway, "did not want to let me go," and that the second plan, offered by WellCare, had yet to send her an insurance card. 

Medicare records show that Mrs. Beer is now enrolled in WellCare's Signature plan. But she continues to receive mail from Gateway certifying that she is a member. And Gateway's automated telephone information system verifies that she still has coverage.

"This is enough to drive any sane person crazy," Mrs. Beer said, expressing frustration at the uncertainty of her situation. 

Pharmacists and beneficiaries around the country report similar experiences. Christopher J. Decker, executive vice president of the Pharmacy Society of Wisconsin, said "it happens fairly routinely" that pharmacists find patients enrolled in two drug plans. 

"The plans are often willing to cover the same prescription for the same beneficiary but charge different co-payments," Mr. Decker said. "This creates confusion and problems for patients and pharmacists alike." 

James M. Maister, a pharmacist in Lutz, Fla., north of Tampa, said that in some cases involving Medicare patients enrolled in two plans, "both plans have automatic deductions scheduled from the patient's Social Security check." 

Health care providers in Florida said they had alerted federal officials because they worried that unscrupulous pharmacists might submit duplicate claims for the same prescription. In an e-mail response, Marisa B. Duarte, an employee in the Atlanta regional office of the federal Medicare agency, said, "We are still working out systems glitches, so having someone enrolled in two plans is common." Federal caseworkers can "verify the exact plan," Ms. Duarte added.

In his radio address on Saturday, President Bush defended the drug benefit. "This new coverage is saving seniors money on their drug premiums," Mr. Bush said. "The typical senior will end up spending about half of what they used to spend on prescription drugs each year."

Under the Medicare drug program, millions of low-income people are entitled to extra help that eliminates their premiums and deductibles and sharply reduces their co-payments. 

But when a beneficiary switches plans, Medicare typically provides information on the person's low-income status to only one of the two plans. The other may still be providing coverage without realizing that the person is entitled to a low-income subsidy. This helps explain why some low-income beneficiaries have received bills for premiums they do not owe - $25 or $35 a month - while others have been charged $40 for a drug at the pharmacy, when their co-payments are not supposed to exceed $5.

Jane-ellen A. Weidanz, the Medicare project manager at the Oregon Department of Human Services, said: "There appears to be a fundamental flaw in the Medicare computer system that transmits information on enrollment of clients and their low-income status. We have clients who changed plans last November. The first plan has information on their low-income status, but the second plan still does not have the information."

The drug benefit is offered by scores of insurers under contract to Medicare. The plans typically cover different drugs.

Jude E. Walsh, pharmacy affairs coordinator for the governor of Maine, said her state had helped 15,000 people switch to plans that would cover more of their medications. "Many of these beneficiaries have been listed as active in two plans at the same time," Ms. Walsh said. "It is hugely confusing."

When a person signs up for a drug plan, the government is supposed to confirm the transaction in a computer-generated report to the insurance company offering the plan. If a person switches plans, Medicare is supposed to send a disenrollment notice to the first plan and a confirmation of enrollment to the second plan. But in some cases, the government mistakenly sent both notices to the second plan, so the first plan did not know that the person had left its rolls.

Moreover, some plans said they had quit processing disenrollments because they could not tell which notices were valid.

Having people enrolled in multiple plans can cause financial headaches for insurers. "Plans have paid claims for beneficiaries who are no longer enrolled in their plan," the Bush administration said in its memorandum to insurers.

The government said it would help insurers determine who should have paid which claims, so they could reimburse one another. If, for example, Plan A paid Medicare drug claims that should have been paid by Plan B, then Plan B owes money to Plan A.

Douglas D. Adkins, the executive director of Dayspring Village, an assisted-living center for people with mental illness in Hilliard, Fla., said the reconciliation process would be difficult.

"It's like trying to undo spaghetti," Mr. Adkins said. "It will take a lot of effort to untangle it all."

More than one in six Pennsylvania residents is on Medicare. For more than 20 years, the state has had its own program to help older adults with their drug costs. Thomas M. Snedden, director of the program, said he was eager to help 200,000 Pennsylvanians enroll in Medicare drug plans that would meet their needs.

But Mr. Snedden said he could not do so because "the federal government cannot yet provide us with a complete, accurate list identifying which state residents are already enrolled in which Medicare drug plans and which people are entitled to low-income subsidies."