How are double spends and "dropped" transactions represented in BlockChain.info API
Double spend transaction (transaction that lost the competition) have no block height. So you can check this field. 

Also you review other approach using callback or webhooks to get update that new transaction arrived. Most blockchain API provide this service.

You can check also our API https://bitaps.com/api/#Create_payment_address

Your task in this approach is only create callback handler.