Thushanka can you post the output of php flarum info so we get your Flarum version and list of extensions?
Do you know how to get in the browser javascript console ? (Developer tools > Console in most browsers) Can you check if there is any javascript error outputted there ?
EDIT: for clarification for other looking at the issue, a duplication of the page elements like that is usually the result of an error in a Mithril component, and the error is usually visible in the console, at least in Chrome. Firefox sometimes can't show those errors. The list of extensions should point out if one can be the culprit.
Also additional question, did you install any extension recently that could have introduced the issue ?
