Hi.
I was wondering what is the most up to date solution for the famous "Double spend" problem?
Is there any solution (commercial company development even..) that reduce the transaction waiting approval time?
The solution is Bitcoin. Double spending is not possible because of the blockchain that seals the transactions and the order of them.
However, those sealed transactions can be leveraged in the form of smart contracts, to support layers like Lightning Network. Lightning Network can be used to send money instantly and 100% confirmed, without the possibility to double spend. But you need to have certain transactions (transactions which enable you to leverage them on layers), sealed in the blockchain.