Signatures that depends only on `msg.sender` are nonce independent.

[rmeissner]

Essentially when the msg.sender is an owner and the Safe has a threshold one, the nonce is never checked. This is because in this case the check on contract level only checks if the msg.sender is an owner, never validating the hash with the nonce.

What can happen in this case?

If we have multiple owners and threshold 1 all could send transactions at the same time, and they would all be executed.