In the Environment View, create a New Variable using the context menu or the toolbar button. Give it the name PATH, the value you desire, and change the Behavior from Replace to Append or Prepend.
1. Will the PATH variable be cleaned up off the values appended on uninstall.
2. How to check if the PATH variable already contains the value to be appended.
1. Yes. Test it yourself, don't take my word for it.
2. Why do you need to? Adding the same dir twice in a PATH var doesn't have any bad effect. Besides, its is kind of a hard thing to do, as the dir must be created by your install and how do you have 2 installs creating the same directory?!