Why a receiver could respnd a retry PUBREL message in MQTT QOS 2?

Assume the Receiver has recieved the PUBREL from the Sender, according to the standard, it would discard the packet id. If the PUBCOMP message does not arrive at the Sender successfully, the Sender would retry to send PUBREL message and when it arrives at the Receiver, the id has been discarded at the first time when PUBREL arrived. Will the sender respond a proper PUBCOMP? This means the receiver still stores the id after gets PUBREL and it is contradictory.

Most articles just tell that the Sender would retry to send but not tell why. Thanks for your kind answer.

For MQTT3.1.1 see MQTT-4.3.3-2:

MUST respond to a PUBREL packet by sending a PUBCOMP packet containing the same Packet Identifier as the PUBREL.

MQTT v5 has similar rules but the PUBREL includes a "PUBREL Reason Code" which can be set to 0x92 to indicate:

The Packet Identifier is not known. This is not an error during recovery, but at other times indicates a mismatch between the Session State on the Client and Server.

In both cases the spec seems quite clear, if you receive a PUBREL you must respond with a PUBCOMP. So the why could be "because the spec says so", but in reality this approach just makes sense (not sending the packet would mean the PUBREL would be resent the next time the connection is reestablished).

This means the receiver still stores the id after gets PUBREL and it is contradictory.

As per the spec the ID comes from the PUBREL packet, not the session state (which has been deleted previously in your example), so there is no contradiction.