The Safe contracts have a so called nonce. This is to ensure that each transaction can only be executed once so no replay attacks are possible. The Safe nonce is equivalent to the nonce of externally owned accounts (EOAs).

In case there are 2 Safe transactions with the same nonce that are waiting to be executed, they are conflicting with each other since only one of them can be executed successfully, thereby invalidating the other one.

An executed Safe transaction increases the current Safe nonce by 1. The transaction afterwards need to use this new, incremented nonce.