You can't double spend it, since a node that has the "old" transaction already in their mempool doesn't accept the "new" transaction.
That is to prevent double spends.
F2Pool has recently
enabled 'replace-by-fee' feature, which allows you to 'double-spend' unconfirmed txs by adding higher fee.
Though most nodes would refuse to accept to mempool and relay a replacement tx until they have this feature enabled (which may
happen in the future).