If a company has a Github repo under GPL license, can they, as the owners of the original code, distribute closed-source copies of it, i.e do they have a different right over the code ?
If you are the sole copyright holder, or if you have the agreement of all copyright holders, then you are not bound by any license. If you are not the sole copyright holder then any copyright holder can sue you for copyright infringement if that is what you do.
And "distribute closed-source copies of it" is not "using it privately". The GPL license allows you many things as long as you don't distribute anything, whether you are copyright holder or not, but as soon as you distribute, you must follow the terms of the GPL license, or you can be sued by any copright holders.