So the bitcoins returned to 1NxmG7uszkcUKgNEbvjrxmvajepMgdVg8S?
More like sent to 1NxmG7uszkcUKgNEbvjrxmvajepMgdVg8S and 1F2r88aWRUo5X3MK7Gs4p2UoH8Yksw4nRC.
How the owner of 1NxmG7uszkcUKgNEbvjrxmvajepMgdVg8S can avoid this situation in the future?
By paying higher fees to make the transaction confirms faster. It seems like that address only ever pays 0.0001BTC for every transaction. The transaction you posted is 667 bytes in size so it's only paying 14 satoshi per byte which is considered very low as of now.
The recommended transaction fee right now is 100 satoshi per byte or 0.001BTC per KB according to https://bitcoinfees.21.co/ . Note that it doesn't mean they'll have to pay 0.001 for every transaction. Transactions have size and most wallets allows you to set the tx fee per KB.
The receiver can also try to avoid it by continuously rebroadcasting the transaction and/or by spending the received bitcoins and using higher fees.
Edit:
And this other transaction https://blockchain.info/tx/3538fa649cecf188fbfd4afc5722b019f57ed9fa28138c3f1ea180f4ac8fcf32 is to other address, with other value an in other date, why this invalidate the first transaction?
Because both transactions used same inputs. It's basically trying to send same bitcoins twice, which is impossible.
See that 1NxmG7uszkcUKgNEbvjrxmvajepMgdVg8S (0.11901 BTC - Output) and 1NxmG7uszkcUKgNEbvjrxmvajepMgdVg8S (0.1095492 BTC - Output) on the left? Those are the inputs.