>> But it can generate a duplicate after 2^15 + 1.
> No, you can't, because the top bit will be different.

Not necessarily - that's why the +1.

|---high bit 0---||---highi bit 1---||---high bit 0---|

If the last value of the first "high bit 0" equals the first value of
the second "high bit 0", it's just generated a duplicate, 2^15+1 from
the first copy to the second.

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