OpenVPN server assigns same IP address (10.8.0.6) to all connected clients

Your logs show that each client connected using the same client certificate, and when that happened OpenVPN dropped the other connection.

Feb 24 15:28:06 openvpn openvpn[590]: MULTI: new connection by client 'client1' will cause previous active sessions by this client to be dropped.  Remember to use the --duplicate-cn option if you want multiple clients using the same certificate or username to concurrently connect.

As a general rule, different users should have different certificates, but if you want to allow the same user to use the same certificate on multiple devices, you can do what it says and start OpenVPN with the --duplicate-cn option. On Ubuntu you can do this by editing the /etc/default/openvpn file and adding the option to OPTARGS.

# Optional arguments to openvpn's command line
OPTARGS=""

would become:

# Optional arguments to openvpn's command line
OPTARGS="--duplicate-cn"

Then restart OpenVPN.