Hi,
There will be one managed identity created per environment and that MI will be associated with all the container apps in that environment, so if you enabled System assigned identity at container app environment , you are not required to create it for the container apps under that same environment
So if you already enabled at container app env level , it is not needed at container apps
Please check doc for ref https://learn.microsoft.com/en-us/azure/container-apps/managed-identity?tabs=portal%2Cdotnet
Kindly accept answer if it helps , Thanks!