%S        Second as a decimal number [00,61].

The range really is 0 to 61; value 60 is valid in timestamps representing leap seconds and value 61 is supported for historical reasons.

The 'double leap second' issue has been around a long time and is part
of the Posix standard (for some background see
http://www.ucolick.org/~sla/leapsecs/onlinebib.html, specifically the
section named 'Unix system time and the POSIX standard').

The "double leap second" is complete nonsense. It never existed outside the 
ANSI C standard and never will. It was introduced by the ANSI C 89 committee 
to document its problems understanding UTC issues. Someone heard that there 
are two prefered dates for leap seconds per year and this got munched up to 
the false rumour that UTC days can be up to 86402 seconds long. The 
definition of UTC, which requires that |UTC-UT1| < 0.9 s all the time, 
obviously makes double leap seconds mathematically impossible.

x = [1,2,3]
y = 'o'

result = [a for b in x for a in (y, b)]

print(result)
>>> ['o', 1, 'o', 2, 'o', 3]