Technically, you can do sublimation over an already sublimated shirt, the fabric will be accepted. Practically, it depends on the pre-print and what you want to add to it. There is a high risk of bleeding or obsessing from the previous print, “picking up” some color from it as you apply a new layer, or mixing new and old colors in a very unclear and unattractive way. You can carefully add new features like names and logos to already printed shirts or fabrics.
Can I put sublimation over sublimation? so, if we give you a direct answer; that process works if the design on your fabric is only one color on a white background. If there are more than one color, then it will not work better.
Can I sublimate a shirt twice?
Yes, you can, but it cannot be guaranteed, because the second press will not remove some of the first print design. In this case, make sure you keep it covered with Teflon or Kraft paper. If it is so, ink doesn’t spread everywhere you don’t want it to. Important thing is fabric type. You may better think about it also.
Can you sublimate twice on a tumbler?

Can you sublimate a tumbler twice? Tumbler can be sublimated any number of times without damaging it. However, you should be aware that when the previous print image is reheated it will appear as a ghost image in the background.
How many times can I use a sublimation transfer?
No. No. No. You can’t reuse any kind of thermal transfer paper. With sublimation paper, you may notice that there are a few inks left on the paper, which may not be enough to make a good quality print.
Does sublimation or heat transfer paper go bad?
They do have an expiration date. It is better to store in a cool, low humidity place. Because the shelf life of every paper is about one year. Anything less than a year from the date of purchase will be deemed obsolete and will have to be replaced if it causes any unresolved transfer issues. On the other hand, if you have a good business, your old paper stock will not remain for months.
Here you can read some manufacturer’s experiences.
Gecko Signs NT
In my experience you can sub or a sub print. I recently produced shirts that contained 19 separate images on it that overlapped to produce a whole shirt design. Yes, there is some ink depreciation when you repress the next image on but it can be managed. Firstly, allow the images that are on to cool completely before pressing the next image. I had a run of 20 shirts to do so that wasn’t a problem.
Main thing I found was that you have to control the cross-ink transfer. Teflon sheets will retain sub ink in them when placed over an existing print and then will re-transfer it to the next garment. Grease proof paper between the Teflon and the exposed inks will stop that, but the paper is for one use ONLY, you will see the ink on the paper.
I am using an artisan1430 with Chinese inks and a cheap CISs system and the standard Epson driver on ICM mode and a 1200mm roll of sub paper cut into 329 wide strips. This allows me to do mass runs, with very high-quality prints and the final product is amazing. I use the same stuff for printing on mugs, neoprene coolers, phone covers, metal etc.
Viper Graphics
My first experience trying to sub for sub was when a customer wanted their name on a mug, I already printed. It does sub back some on the paper and fades a bit. Because, first sublimated image may deteriorate a bit, like being exposed to the heat again made some of the ink fade or “disappear!” not a lot but you can tell. I normally wouldn’t have sold that mug but the customer thought it looked great. In this case I agree the customer is always right.