It's easy to find bears claiming this stock or that stock is wildly overpriced. Usually it's opinion—but not with the newly listed New York shares of SK Hynix. Earlier this month, the chip maker introduced American depositary receipts to add to its longstanding listing in South Korea.

Its new ADRs are each backed by one-tenth of a Seoul-listed share and can be converted into a Korean share with little effort. No one would choose to convert, though, because the U.S. stocks trade at a fat premium to the price in Korea, after adjusting for currency. It's yet another sign of the frothiness of the AI trade.

Since the ADRs listed two weeks ago, the premium has ranged from 16% to 51%. Americans are paying a high price for the convenience of buying the memory-chip maker's stock in New York, rather than finding a broker willing to trade the Korean shares. The huge ADR premium is exactly the sort of thing that shouldn't happen in markets.

Shares are virtual and can be moved instantaneously, so arbitragers usually step in and profit from any significant gaps between dual-listed stocks.