ravi sir,I am not asking for healthy practice, i want to know whether it is allowed or not under income tax act

i think it is allowed if u give reasonable explanation

If you have a original invoice then definitely allowed.

If you do not have original invoice but have a duplicate - then you are at some kind of risk. 

But during any assessment or scrutiny the officer may not check each and every invoice unless he has a doubt that some expenditure is claimed wrongly.

For E.G in construction of building you have shown only cement purchase and there is very little purchase of sand etc. 

Expenses will be allowed on the basis of duplicate invoice..

allowed in income tax .get is stamped as CTC and signed by the issuer of invoice

I totally agree with the reply of CA Praveen.Nice and accurate answer is given by him. 

You have to get it certified by the issuer of Invoice ,if and only if original Invoice is not traceable.

whether if  authorised signature and seal affixed in duplicate copy of invocie it is ok otherwise it is not a allowable expenditure.

If Duplicate copy is issued by the same person with authorised signatory(with stamp), then it can be considered.

Don't Worry.  Its Allowed