How to remove double spacing

https://www.ads-software.com/extend/plugins/remove-double-space/

https://en.forums.wordpress.com/topic/remove-double-spacing

The plug in doesn’t seem to do anything and and everything else I do removes the double spacing but also removes any spacing.

I want it too look like this

Test line one.
Test line two.

But it ends up like

Test line one. Test line two.