To manually add or remove terms to/from a localization project, you need to go that project's Terms page, where you can find an editable list with all the terms in your localization project.
How to add terms to a 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.
How to remove terms from a localization project
To remove one or more terms from a software localization project, 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 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.
The only way to have term duplicates in a POEditor project is by adding identical terms with different context attirbutes.