One country, two (or three) UN seats

Next month Taiwan will again attempt to get accepted to UN membership, and once again China's overwhelming influence will no doubt defeat it. But China is not being consistent here: it argues that Taiwan, which it considers as part of China, is already represented through the PRC's membership. But back in the days of the Soviet Union, Ukraine and Belarus (then known in English as Byelorussia) had their own UN seats in addition to being represented as constituent republics of the Soviet Union. We didn't hear China complain about double representation back then, so why now?