All taxes shall be uniform upon the same class of subjects within the territorial limits, of the authority levying the tax, and shall be levied and collected under general laws, which shall prescribe such regulations as shall secure a just valuation for taxation of all property, real and personal: provided, that the legislature may allow such exemptions from taxation from time to time as shall seem necessary and just, and all existing exemptions provided by the laws of the territory, shall continue until changed by the legislature of the state: provided further, that duplicate taxation of property for the same purpose during the same year, is hereby prohibited.