You can legally dual license if you have ownership over all the code in question. This is common for open source industrial software; pay for a different licens

You can legally dual license if you have ownership over all the code in question. This is common for open source industrial software; pay for a different license so you can embed it in a closed source project. The GPL doesn’t restrict you from offering the same code with a different license if you own it. Often the issue is that some projects don’t require contributors to sign over copyright ownership as part of contributing. So you have a project that’s licensed uniformly, but each contributor still owns their individual contribution. Unwinding this after the fact can be a nightmare, as it involves either finding every contributor and asking them to sign over their code, or manually removing every bit of code you don’t own as a project. This is why a lot of bigger projects require you sign a contributors agreement that assigns copyright before you can contribute to the main repo. Doing this in advance saves the project a lot of headaches down the road if dual licensing is deemed useful. This is true even if you want to license under two different open source licenses, as only the copyright holder can change the license.