If you mean as an investment then you pay investor SD rates.
If you mean a new PPR then if the property was new and under 125sqm then no SD otherwise second time buyer of a second hand house SD rates which are the same as investor rates. See CitizensInformation for more details.
Stamp duty on the second property you purchase would depend on whether that property is a new build, the size of the property and whether or not you are going to be an owner-occupier of it.
Of course there is then the question of a clawback of any relief you got on the first property if you are no longer going to live in it.