You can offer your code under as many licences as you want. So you can publish the same code under both GPL and a proprietary license. This is called dual-licen

You can offer your code under as many licences as you want. So you can publish the same code under both GPL and a proprietary license. This is called dual-licensing. The people that receive the code from you under one of those licences will not have that liberty. They will have to abide to the licence you gave them. For proprietary licences, that usually means they are not allowed to do more than use it. For GPL, that means they can pass it on under the GPL. So in your example, while people could share the code you publish under the GPL, it does not impact what you can do with your closed source version. Only when you'd start to integrate patches to the GPL branch from third parties, you'd get into trouble.