Question

aisi kaun si chiz hai jo hafte me do bar aur saal me ek baar

Answer 1

week - there are 2 'e' in the word week
and
year - there is only one 'e' in the word year
thus, the thing which occurs once in a year and twice in a week is the letter 'e'

Answer 2

$\large{\mathfrak{Answer -}}$

$\textbf{\underline{Letter "e"}}$

It's the letter "e" which comes once in year and twice in week.