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Hello. I'm sorry you keep going through this.
I am very surprised that they have gone back in and she's still showing heat signs. This is highly unusual. I would probably get in touch with a veterinary surgeon to go in the 3rd time. It's safe to go back in, but you want to make sure it's someone that is skilled at finding these tiny remnants and removing it all. I would also double check the adrenal glands, as some adrenal tumors can look like ovarian remnants. That would be much less common, but still worth examining.
You definitely want to have the surgery done. This will prevent estrogen derived tumors (mammary) or diseases (stump pyometra) from occurring in the future.
I hope this helps, let me know if you have any other questions.