Go to the project page and click on the View or Add Terms button in the side menu.
You'll be taken to a page containing all the terms in your software localization project. You can manually add a term by clicking on the Add term button at the bottom of the page, typing it and pressing Save.
To remove one or more terms from a software localization project, in the View or Add Terms page, check the boxes next to each string you want to delete, choose Delete Selection in the drop-down menu at the bottom of the page and press Go.
In View or Add Terms you can also see and add comments, add plural forms, see all the translations in all the languages made for a term, or add/edit reference and context.
Note: The only way to have duplicates in a POEditor software localization project is by importing identical terms with different contexts.