Someone else is selling on your listing, possibly counterfeit, possibly just unauthorized. Here's how to deal with it.
A "hijacker" is another seller who adds an offer to your existing product listing, sometimes reselling genuine product, sometimes selling counterfeits, sometimes just undercutting your price to win the Buy Box.
If you're not enrolled in Amazon Brand Registry, this is the single highest-leverage move against hijackers. Transparency codes make it far harder for anyone to sell counterfeit units under your listing at all.
If you're not sure whether you qualify for Brand Registry, this is usually a same-week fix that pays for itself the first time it stops a hijacker.
Persistent hijackers sometimes require an escalated, documented pattern of test-buys and reports before Amazon takes lasting action.