A large stamp duty was one successful house price cooling measure used by the Singaporean government recently. They exempted first-home buyers and buyers who sell their existing house.
Evidence suggests it works. The most the prices would drop is the difference between the speculation-supported price and the price that first-home buyers would want to pay. People who currently have a lot of houses would find it difficult to sell their current houses at the price they bought/borrowed for them, but that's a risk they were willing to take.
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