Hello,
Could you provide more details?
Do two different devices receive the same IP or DHCP lease is simply repeated twice in the table?
Does the RUT device have DHCP server enabled?
If yes, does the configured IP pool overlap with the static IPs?
Could you attach a troubleshoot file to your question to view your configuration? Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.
Best regards,