Spins it is an interesting statistic, although I cannot offer any verification, the Rule of Thirds seems to build as it moves toward a complete cycle. Thus :-
In 12 spins 2 repeats(should be 4)
In 18 spins 4-5 repeats (6)
in 24 spins 7-8 repeats (8+)
in 30 spins 10 repeats (10)
in 37 spins 13-14 repeats (12+)
I have rarely used single numbers so I can't vouch for this. It does seem to have merit and offer possibilities for exploitation.
You misinterpret law of thirds, if what you wrote was true then with the same logic we would see within any 3 spins cycle 1 repeat, of course this is far away from truth.
The explanation is quite simple, the proportion, or ratio if you prefer, between "repeaters" and "sleepers" is not fixed but dynamic.
In other words the more numbers have appeared, the more repeaters we should expect, as one category increases so does the other proportionally.
A good indicator could be the first third, I mean the first 12 spins, if "law of thirds" would was a fixed value, then we should encounter 4 repeaters within first 12 spins, this is not impossible but it's not the average expectation.
I consider average 1 to 2 repeats within the first 12 spins, of course none is possible too but not what usually happens.
My main focus is in what happens the majority of times and not for the exceptions of the rule.
So if you encounter 0 or 3,4,5 (5 repeats within first 12 spins most I've seen) repeaters within 12 first spins, it could be an indication for fewer or more frequent repeats in the next 24 spins.
0 repeats is not so rare as is 4 or 5 repeats within first 12 spins.
The fewer the repeats on the first third, the fewer will be on the following 24 spins, the more the repeats, then more frequent within next 24 spins.
Speaking in numbers, consider 2 repeats as the most average result, thus an indication for 22 up to 24 unique numbers and 15 up to 13 repeats/sleepers.
Consider 1 repeat as indicator for 24 to 25 different numbers and 13 to 12 repeats/sleepers.
You should think in proportions / ratios terms.