The basic rule
When you're covered by two plans, one is primary and one is secondary. The primary plan pays first. A secondary plan with a non-duplication clause then calculates what IT would have paid if it had been primary, subtracts what the primary already paid, and pays only the difference.
Why it often pays nothing
If the primary plan paid more than the secondary's allowed amount, the secondary owes $0. Example: a $1,000 service. Primary pays $800. Secondary (non-duplication) would have paid $700 as primary — $700 minus $800 already paid = $0 from the secondary.
How it differs from traditional COB
Traditional Coordination of Benefits often pays up to 100% of the allowed charge — covering what primary left behind. Non-duplication is stricter and usually leaves you responsible for the remaining cost-share.
What to check
Look in each plan's Summary of Benefits or Evidence of Coverage for the words 'non-duplication,' 'maintenance of benefits,' or 'COB.' If the secondary is non-duplicating, don't assume your out-of-pocket will drop to zero just because you have two plans.
Birthday rule for kids
When children are covered by both parents, the parent whose birthday falls earlier in the calendar year is usually primary. The date of birth — not the year — is what matters.