Why was this transaction accepted? In testnet, txid:fb7a8658ec015133e36e2cf7ddf7e8c887c3a5becec2f30f24ebfe43e72f4b59
transaction has two inputs.
This two inputs indicate the same txid and vout,
6a08723bc717e1ddf91fa60fde25784ef66952e8687f3bffe391fc2c819dbfd9:1.

Is this a double spending?
This transaction attempts to exploit the recently disclosed (and fixed) vulnerability regarding the validation of duplicate inputs in a single transaction.

There is an excellent answer on this question on how that vulnerability works.