Situation 

The target server has a policy that blocks duplicate IP addresses. When connecting via Client VPN, all source IPs appear as the VPN Endpoint IP, so multiple users cannot access the server simultaneously.

Is it possible to create multiple Client VPN Endpoints so that each user can access the server using a different Endpoint IP?

Cause 

Client VPN uses NAT to change the source IP to an internal VPC IP when accessing the VPC. Although the source IP changed by NAT may differ per associated subnet, it cannot be set differently for each user. 

Solution 

To allow users to access with different source IPs using VPN, you must use Site-to-Site VPN. Client VPN does not currently provide this functionality.

Reference : [1] What is AWS Site-to-Site VPN? https://docs.aws.amazon.com/vpn/latest/s2svpn/VPC_VPN.html