When reproduction fails, checking the same nightly when the start day and end day are the same

If the start day and end day are the same when reproduction fails, check for duplicate nightly.

code

#![allow(incomplete_features, unused)]
#![feature(min_generic_const_args)]
#![feature(inherent_associated_types)]
struct Foo<'a> {
    x: &'a (),
}

impl<'a> Foo<'a> {
    #[type_const]
    const X: usize = 1;
    fn foo(_: [u8; Foo::X]) {}
}

fn main() {}

command

cargo bisect-rustc --start 2026-01-09 --end 2026-01-09 --regress=non-ice

log

checking the start range to find a passing nightly
installing nightly-2026-01-09
rust-std-nightly-x86_64-pc-windows-msvc: 21.22 MB / 21.22 MB [===========================================================================] 100.00 % 9.07 MB/s testing...
RESULT: nightly-2026-01-09, ===> Found ICE
uninstalling nightly-2026-01-09

checking the end range to verify it does not pass
installing nightly-2026-01-09
rust-std-nightly-x86_64-pc-windows-msvc: 21.22 MB / 21.22 MB [==========================================================================] 100.00 % 10.70 MB/s testing...
RESULT: nightly-2026-01-09, ===> Found ICE
uninstalling nightly-2026-01-09

ERROR: the end of the range (nightly-2026-01-09) does not reproduce the regression

Start by running the reported cargo bisect-rustc command with identical --start and --end dates, then trace the code handling the passing-range and regression checks. Verify the same nightly is not installed and tested twice, and confirm the command reports the regression result correctly for a one-day range.