if multiple `-Z allow-features` are passed, only the last is used

I tried this code:

rustc +nightly -Zallow-features=ffi_pure -Z allow-features=rustc_attrs src/main.rs

I expected to see this happen: A hard error. It does not make sense to combine allow-features; should they be OR-d or AND-ed?

Instead, this happened: Neither an OR nor an AND. Instead the first flag was ignored altogether.

error[E0725]: the feature `ffi_pure` is not in the list of allowed features
 --> src/main.rs:2:12
  |
2 | #![feature(ffi_pure, rustc_private)]
  |            ^^^^^^^^

Meta

rustc --version --verbose: rustc 1.88.0-nightly (78f2104e3 2025-04-16)

Reproduce the issue with the shown rustc command and inspect how repeated -Z allow-features arguments are parsed. Establish whether repeated values should be combined or rejected, then verify that the chosen behavior handles both features and no longer silently ignores the first flag.