I'd like to use code from project A that was licensed under GPL-3.0-or-later in project B, which is also going to be licensed under GPL-3.0-or-later. What should the license headers in the source files look like?

I found a detailed description of how to cite permissively licensed code here: https://softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.html. The suggestion is to include the following phrase in the header:

This file incorporates work covered by the following copyright and permission notice:


This seems like overkill if I'm using GPL-licensed code in a GPL-licensed project (I'd have to put the GPL-3.0-or-later license header into each source file twice). Is there a better way to do it, or should I still follow this pattern? What should the license header look like?

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The document you link to is specifically about best-practice if you are combining works under different licences (where the licences permit this combination). You aren't doing that; you are combining two works under identical licences.

You will need to maintain all existing copyright notices, adding your own in any files that you have added copyrightable content to. But you don't need to duplicate the licence notice itself, since the resulting work is under a single set of licence conditions (GPLv3+).

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