Both are correct. Twice possibly more common in British (and Australian) English, "two times" US English.
Well "Once" means one time. For example: "once upon a time..."
You should only use "is" once, not twice in a row.
opportunity comes but once
My current school timetable has French twice a week and German once a week.
The prefix is bis- and comes from the French 'biscuit' meaning 'to be cooked twice' (bis-cuit). The original biscuit was a slice of bread that was baked (or toasted) another time (because the bread was already baked before being cut into slices). It's the same as the German word 'zwieback' (twice baked) or the Dutch word 'beschuit' (obviously derived from the French word)The actual prefix is "bis", an Italian prefix meaning "twice". The root word "cuit" means "cooked". Biscuits are cookies that are baked twice, which makes them crispy.
The letter 'S'. Saturday and Sunday come twice a week and September comes once a year..
The letter 'e'
The letter a
The letter 'e'. It appears once in year, twice in week and never in day.
The letter 'e'. It appears once in year, twice in week and never in day.
The letter E appears twice in the word "week" and only once in the word "year."
I
the letter "A"
M
vowel
the letter "e"
The letter 'n'