cost of acquision

You cannot claim interest of home loan if you have taken benefits of same u/s 24.
Also you can add to cost of acquisition only if it is capitalised every year, for that check relevant AS whether you can capitalise it or not.

but some case laws allowed deduction under both heads.

You will have to read case law in detail and know the exact facts of the case. If it matches with your case then take the benefit and rely on the case law. Also check whether there is any contravening case law against the same by upper authority and latest one.

Because taking benefit of interest u/s 24 and also adding the same to cost of acquisition shall tantamount to dual benefit for same transaction and which shall not be allowed.

taking deduction under section 24 us related to your house property head while claiming deduction towards interest paid is taken under capital gains.section 45does not put any such restrictions that if deduction is allowed under section 24 same will be disallowed under capital gains. the two are altogether independent section and each if them does not bear any relevance on each other.
hence you can claim deduction of interest paid on home loan as COA

Girdhar Sir, any documentary evidence in support of your claim of interest in both the sections?

refer the respective sections of the act

No, that amounts to double exemption. Provide any case law or any notification in its support, to produce the same before AO, as in one of such case AO has rejected the capital gains.