Combine existing GitHub pull requests into one commit
By the "receiver" of the pull request
If you would like to see only one entry for the Pull Request in your project's history, then please enable this GitHub functionality on your repo. It allows you to squash (combine) the commits when merging.
By the "sender" of the pull request
Unfortunately, the "sender" of the pull request does not have a button in the GitHub GUI to squash the commits in an existing pull request. Instead, they need to to it by hand using the command line, which is unfortunately cumbersome but can be done like this: