if you look majority are only installed inboard only, because fan cowls can be fitted either side of an engine it can be un-noticed defect. I think some a/c such as an A319 may have fins either side though.
You will find strakes on both sides on all A319 aircraft and on A320 LIP (Lift Improvement Package) aircraft. This is also the easiest way to distinguish a standard A320 from a A320 LIP.
This extra engine strake together with several modifications on the leading and trailing edge of the aircraft enables you approach about 6kt to 7kt slower than the standard aircraft.