The reason he resolved to sue was not just because of Tang Fangjing's negligence, but also Old Niu's confidence.
If you ask whether there had been cases before of developers refunding double the deposit, of course, there were.
After all, in the past, those developers had too many tricks up their sleeves—they would start pre-sales without even obtaining the pre-sale permits, or they would sell one property to two different buyers. There really were many such cases.
In those situations, as long as you sued, you were assured a double refund of the deposit, but many people were not aware of the issues then, and developers' contracts would often include clauses like refunding the deposit in case the developer breached the agreement...
So, in cases where the developers voluntarily refunded the deposit, many chose to settle things peacefully.
However, unless it was a matter of "principle," courts would not award a double refund. There had not been a single case like that!