What do you do if you discover that some part of your own open source project infringes copyright?
Consider the following hypothetical situation. An open source project's maintainer finds code in his Github project which infringes copyright. i.e., There is code which he was responsible for which wasn't legally copied into the project. What should be the procedure to remedy it?
If you find, or are notified, that you are infringing on the copyright of someone else, you have a few options.
If the infringing code is under a copyright license that is compatible with the rest of your code and that has terms you can agree with, then you could re-add the copyright and license statements that were erroneously removed.
In all other cases, our only option is to remove the infringing code from your codebase. This will likely remove some features or break the application in other ways, but that is a consequence you will have to pay. However, you are allowed to add the features again, as long as you don't copy code without the proper permissions.
Your action should be as Bart van Ingen Schenau described. Actually, exactly the same action that you should take if that happened in your closed source software.
However, it is much more important than in closed source software to be clean. Closed source is usually not distributed, so if there are problems due to infringing code, that hurts that one company only. And it only hurts those who deserve to be hurt.
With open source software, it may be widely distributed. So any infringing code could end up with thousands of developers. And when the infringement is found (by the copyright holder) and you haven't acted promptly, your inaction could have bad consequences for all these thousands of developers, through no fault of their own.
