I know this is not the proper way of storing data
Phew!!! That is an understatement!!!
So do you have duplicates only on contact_number? If so, you could do something like this:
select ID, MAX(name), MAX(age), x.contact_number
from #temp
cross apply (select contact_number from #temp) x
where x.contact_number is not null
group by ID, x.contact_number
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