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Why companies appear in several stock exchanges?
If I buy the one from NSY, is it the "real" Sinopec? No - you are buying an American Depository Receipt. Essentially some American bank or other entity holds a bunch of Sinopec stock and issues certificates to the American exchange that American investors can trade.  This insulates the American investors from the cost of international transactions.   The price of these ADRs should mimic the price of the underlying stock (including changes the currency exchange rate) otherwise an arbitrage opportunity would exist.  Other than that, the main difference between holding the ADR and the actual stock is that ADRs do not have voting rights. So if that is not important to you then for all intents and purposes trading the ADR would be the same as trading the underlying stock.