Generate keys lets you generate keys for account ids already existing

[H34D]

Describe the bug
When i create keys for an account id that is already known to the network it will still create keys

To Reproduce

$ export NEAR_ENV=testnet
$ near generate-key acc.testnet
Key pair with ed25519:74CzZ61AY1W7jsUEMJRJYiZxK1jWkUfM43qD2fZid4mg public key for an account "acc.testnet"
$ near keys acc.testnet
Keys for account acc.testnet
[
{
access_key: 

{ nonce: 9, permission: 'FullAccess' } 

,
public_key: 'ed25519:9VSwjGM1Qx4yHTaVNGqkng994cTHnzVqZpS2MGPUegFs'
}
]

Expected behavior
A warning like on the wallet app:

Desktop (please complete the following information):

• OS: macos
• Version 11.5.2