Performing a stable sort on a repeated pair of keys that are already in sorted order causes the following error message.

core.exception.AssertError@std.algorithm(3736): Assertion failure

However, doing so with repeated reverse-sorted keys works fine.

In other words, a stable sort of the array
[ "B", "A", "B", A" ] will succeed, but one of ["A", "B", "A", "B" ] will fail.

Illustrative code:

import std.algorithm;
import std.stdio;

void main()
{
    auto ar1 = [ "B", "A", "B", "A" ];
    sort!("a <  b", SwapStrategy.stable)(ar1); // ok
    writeln(ar1);

    auto ar2 = [ "A", "B", "A", "B" ];
    sort!("a <  b", SwapStrategy.stable)(ar2); // fails
    writeln(ar2);
}