Fix for init source kwarg alias resolution.

pydantic_settings/sources.py

@@ -388,7 +388,17 @@ def __init__(
         init_kwargs: dict[str, Any],
         nested_model_default_partial_update: bool | None = None,
     ):
-        self.init_kwargs = init_kwargs
+        self.init_kwargs = {}
+        init_kwarg_names = set(init_kwargs.keys())
+        for field_name, field_info in settings_cls.model_fields.items():
+            alias_names, *_ = _get_alias_names(field_name, field_info)
+            init_kwarg_name = init_kwarg_names & set(alias_names)
+            if init_kwarg_name:
+                preferred_alias = alias_names[0]
+                init_kwarg_names -= init_kwarg_name
+                self.init_kwargs[preferred_alias] = init_kwargs[init_kwarg_name.pop()]
+        self.init_kwargs.update({key: val for key, val in init_kwargs.items() if key in init_kwarg_names})
+
         super().__init__(settings_cls)
         self.nested_model_default_partial_update = (
             nested_model_default_partial_update

tests/test_settings.py

@@ -662,6 +662,17 @@ def settings_customise_sources(
     assert s.model_dump() == s_final
 
 
+def test_alias_resolution_init_source(env):
+    class Example(BaseSettings):
+        model_config = SettingsConfigDict(env_prefix='PREFIX')
+
+        name: str
+        last_name: str = Field(validation_alias=AliasChoices('PREFIX_LAST_NAME', 'PREFIX_SURNAME'))
+
+    env.set('PREFIX_SURNAME', 'smith')
+    assert Example(name='john', PREFIX_SURNAME='doe').model_dump() == {'name': 'john', 'last_name': 'doe'}
+
+
 def test_alias_nested_model_default_partial_update():
     class SubModel(BaseModel):
         v1: str = 'default'