Around the time when Sendul found the simplest way of duplicating (here), it was entirely harmless to duplicate items. I could duplicate to my hearts content. Perhaps the only limit is the limit of the data type itself.
But then few months later, that method is found to be risky. I was notified when I duplicated too much item. I was greedy then.
So I decided to do some experiment. If I could pick up freely items form searching, why can't I pick up items laying on the ground? But then it was not about dropping or picking, but about changing of value. From Sendul's method, I know that 0.08 and 0.06 (both DOUBLE) corresponds to 2 and one. So, its easy to guess value for 3, 4, 5, etc. And, I checked them by picking one by one and looked at its value on GG. Then, when I had 9 items (0.22 in DOUBLE) and pick another, the value became 0.039. The pattern changed. There was another value that changed but I didn't notice then. So I find what Sraksis has found before (here).
Since then I duplicated items with more careful (but still greedy) method: only duplicate to the highest number of that digit. From 2 duplicate into 9; from 9 pick one item into 10; from 10 duplicate into 99; from 99 pick one item into 100; from 100 duplicate into 999, etc.
Later, I know that this is can be done by not only changing the value of the amount but also the order, as pointed by die440. When I have 9 and pick one, the order is also changed automatically.
SO in conclusion, there are two method of duplicating without getting notified or banned (except by being reported):
1. Duplicate onlly to the highest number of that digit, and then drop/pick up to increase digit.
2. By changing the DOUBLEs correspond to the amount and the order.
The amount --> ######A0 0.08D
The order --> ######B0 2.0D
If we want for example 7500 units, then change that number into 0.37525D and 5.0D.