Our web application is being mimicked by another domain ...
The domain in question is configured to resolve to the same IP address as yours. That's why it looks like they mimic you when in fact it is simply the same physical server, only accessed by another name.
But when using the URL from your question one gets a security warning in the browser: the certificate is for nrnsewa.com which does not match the name in the URL www.djjpl.com.sg. Since this warning will scare away most visitors I doubt that this is an attack but believe that it is simply a misconfiguration for www.djjpl.com.sg.
How can we prevent it?
You have no control what others configure for their domain. This includes misconfigurations where they accidentally configure the wrong IP address for their site in DNS. This includes also that they don't update their records if they no longer use a domain which means it might point to IP addresses which are used by others in the mean time. Anybody could configure their domain to point to an arbitrary IP address (including yours), both deliberately and by accident.
But what you can do is refuse access to your site or show an error when the domain name in the TLS handshake (i.e. HTTPS-URL's) or the HTTP Host header do not match your site. How this is done depends on the specific server implementation you have. See for example How can I block requests with the wrong Host header set?. And even if you don't care about such misconfigurations, enforcing the correct Host header is still recommended since it prevents some attacks like DNS rebinding.