title: Pull requests overriding earlier commits
body: <p>I am new to Git so bear with me.</p>
  
  <p>I have a Git repository (on GitHub) and I have lots of coders who are inexperienced with Git. When I hire them, I ask them to make a pull request towards my repository so I can review their changes before I merge. At the moment my biggest problem is that often I merge changes into my repository after they fork it, but before they make their pull request, meaning that merging their pull request overrides an earlier commit.</p>
  
  <p>Is there a way for me to ensure that this will not happen apart from telling them to avoid the problem?</p>