PO_ReceiptReturnMatRqByPO is a good place to check.

So I see the batch of invoices that need to be printed the one which is giving error... so is there something else I particular need to make sure of.. 

That table contains records for unposted PO related transactions.  If you see multiples there, pointing at the same PO, that is one possible reason for the error.

Went through invoices in file each one points to its own PO, no multiple invoices to one PO..  

Duplicates in the same batch are impossible, but there can be duplicates in other batches / other PO transaction types.  Read the error again.

So found an article that mentioned the PO_ReceiptVarianceCalculation table I discovered there is one of the PO's in this table that is also in the PO_ReceiptReturnMatRqByPO ... The article mentioned deleting the one in VarianceCalculation but wanted your take on it.. 

If you have legitimate unposted transactions, don't go bypassing system protections by deleting anything.

If the references in those tables are corrupted / orphaned records (and do not represent current unposted transaction data), then the article you found is likely to be applicable.

I did end up deleting the entry in PO_ReceiptVarianceCalculation then ran rebuild for PO and this seemed to fix the issue. 

Thanks for you help...