a(1)=1, then a(n)=3*a(n-1) if n is already in the sequence, a(n)=2*a(n-1) otherwise.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =24a(5) =48a(6) =96a(7) =192a(8) =384a(9) =768a(10) =1536a(11) =4608a(12) =9216a(13) =18432a(14) =36864a(15) =73728a(16) =147456a(17) =294912a(18) =589824a(19) =1179648a(20) =2359296a(21) =4718592a(22) =9437184a(23) =28311552a(24) =56623104a(25) =113246208a(26) =226492416a(27) =452984832a(28) =905969664a(29) =1811939328