meson setup crashes with --backend=xcode

[seishun]

Describe the bug
Configuring a build directory with the Xcode backend for this particular project crashes Meson with the following output:

Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/mesonmain.py", line 193, in run
    return options.run_func(options)
           ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/msetup.py", line 365, in run
    app.generate()
    ~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/msetup.py", line 188, in generate
    return self._generate(env, capture, vslite_ctx)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/msetup.py", line 253, in _generate
    captured_compile_args = intr.backend.generate(capture, vslite_ctx)
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/backend/xcodebackend.py", line 329, in generate
    self.generate_target_dependency_map()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/site-packages/mesonbuild/backend/xcodebackend.py", line 544, in generate_target_dependency_map
    assert k not in self.target_dependency_map
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

ERROR: Unhandled python exception

    This is a Meson bug and should be reported!

To Reproduce

1. Check out https://github.com/TypesettingTools/Aegisub.git at v3.4.0
2. Run meson setup build-xcode --backend=xcode

Expected behavior
No crash.

system parameters

• macOS Sonoma
• Python 3.13.1
• meson 1.6.1 and HEAD