A technique that I've used while fuzz testing string algorithms is to run all tests first with small alphabets (perhaps 4-6 characters), then with full alphabets. It seems to increase the probability of finding bugs with relatively small runs.
My work is on a personal project, not algorithms that have been in production for years, so it's possible that this wouldn't be that productive.
Indeed, the randomized test in this PR uses alphabets from AB to ABCDEF, because I noticed the same thing - small alphabets make repetitions more likely, which are the tricky cases.
My work is on a personal project, not algorithms that have been in production for years, so it's possible that this wouldn't be that productive.