@mndpsingh287
First issue: Your readme.txt says Stable tag: 2.5
, but you don’t have a folder /tags/2.5, so better change to ‘Stable tag: trunk’
Then, as Laurent indicated, you declare Text Domain: duplicate_page
, but in your code you use load_plugin_textdomain('duplicate-page',...)
An underscore is not a hyphen. The text_domain needs to be the same as your plugin slug, so duplicate-page it should be ??
So with those 2 changes, you should be good to go for translation on