Health insurance, it is widely agreed, is a good thing. Can one have too much of a good thing? You bet.

Nearly 6 million elderly Americans, worried about the ruinous costs of long illnesses, have bought two or more policies that supplement their Medicare coverage, a practice that that is unnecessary and costly.Federal and state laws set a minimum level of benefits that a private, so-called Medigap policy must pay. One such policy is all virtually anyone needs. Unfortunately, unscrupulous insurance sales people exist who sell unsuspecting, frightened or confused elderly persons multiple policies.

James Firman, president of United Seniors Health Cooperative, a research group, said 20 to 35 percent of the elderly had some duplicative or wasted coverage.

"We have seen dramatic cases of people spending $5,000 to $7,000 on duplicative and unnecessary coverage. These are invariably people who can least afford it."

A 1980 federal law makes it a crime for an insurance agent knowingly to sell a policy duplicating coverage the buyer already has. No one has ever been indicted under the law. Some agents get around it by carefully not asking customers if they already have Medigap policies.

The House Energy and Commerce Committee plans hearings in April on the problems of duplicative policies.

But more than that is necessary. State insurance commissions, better business groups, consumer protection agencies, advocates for the elderly and, above all, families must get the message across to those over 65: "Usually one policy is enough."