As the author of a codebase, I understand that I have the right to decide how to license my code.
If I write a library of code and release it under the pure GPL, that means that anyone else using my library of code must release their entire project under the GPL (the LGPL would prevent that requirement, but for this argument I'm using the normal GPL).
What if I then use that same library of code in a closed-source, proprietary program that I am selling commercially? Naturally, I would not include any mention of the fact that the code is otherwise released under the GPL.
Naturally, for anyone else, doing that would be against the GPL, but since it's my code, can I legally do it? My immediate assumption would be yes, since it's my own code, but would like to hear if there's any comments on that scenario.
A similar example that I believe I've actually seen done in practice is "community vs. commercial" versions of the same program, with the community version being released as open source and the commercial version being proprietary. However I can't immediately say I've seen this in a situation where the community version is GPL licensed. (It's usually MIT or something similar, which already permits use in closed-source proprietary software.)