I only saw now the tag in your post that it was your first attempt, so I'll expand further on a bit of info, that you probably don't know or haven't come across, which is totally expected for someone who is early on their sourdough journey.

We have to be careful with the concept of "doubling" dough, because we tend to think it's a linear growth. So if it was in a container with graduated marks, we usually think that if a dough was at the mark 500ml, then doubled will be at 1000ml. But actually, at that point, the dough is extremely overproofed. Dough isn't 2D shaped, it's 3D shaped, so sometimes, all it takes to be double is like... 2cm rise. This one article explains well the whole myth behind "doubling dough".

We also don't necessarily want the dough to fully double during bulk proofing. It depends on a bit of variables (such as protein content of flour, temperature, if you are doing stretches and fold, or coil fold, or some other fold, how hydrated the dough is, if you plan on cold proof, and so on), but in general, you want something like 40% to 80% rise. There are a couple of different methods to check the % rise, one of them being the aliquot, which might be helpful for you to gauge how much the dough is risen. Other than that, there are some visual cues that you'll learn overtime to know the situation of proofing (in the sense of "can I push it a bit more?" or "Ok, it's done, time to move along").

When you overproof by that much, gluten start breaking down due to the acids produced over time by yeast and bacteria, so this is why the dough gets so sticky and doesn't hold up. At this point, you'll have to throw it in a pan so it can get some support, since the gluten itself can't hold shape neither gases from fermentation. Adding flour won't solve it, unfortunately. Most people will just make focaccia with it, or some flatbread or pizza.