How we can connect Multiple devices with same certificate and same things with AWS MQTT

Hi. It's a best practice for every device to have a unique certificate: https://docs.aws.amazon.com/iot/latest/developerguide/x509-client-certs.html

We recommend that each device or client be given a unique certificate to enable fine-grained client management actions, including certificate revocation.

AWS IoT offers several provisioning mechanisms that can help you provision unique certficiates: https://docs.aws.amazon.com/whitepapers/latest/device-manufacturing-provisioning/device-manufacturing-provisioning.html

need to use same things for multiple device

Why? Every device will need to connect using a unique MQTT client ID. If a second device connects with the same client ID as the first, the first device will be disconnected. Typically, the MQTT client ID will match the Thing name, as this helps in writing IoT policies that can scale using Thing policy variables: https://docs.aws.amazon.com/iot/latest/developerguide/thing-policy-variables.html

Do your devices have a unique ID, like a serial number, MAC address, or an MCU ID that your firmware can read? Client IDs and thing names are often automically formed at scale using this kind of information.

You are learning that your policies need to adapt to scale, ie. 1 device vs. 10 devices vs. 1000 devices, etc. If you want to experiment with policies, check out our hands-on AWS IoT lab https://mqttlab.iotsim.io/aws/