Add rule for duplicate field names (#6)

src/flake8_pydantic/errors.py

@@ -48,6 +48,11 @@ class PYD005(Error):
     message = "Field name overrides annotation"
 
 
+class PYD006(Error):
+    error_code = "PYD005"
+    message = "Duplicate field name"
+
+
 class PYD010(Error):
     error_code = "PYD010"
     message = "Usage of __pydantic_config__"

src/flake8_pydantic/visitor.py

@@ -6,7 +6,7 @@
 
 from ._compat import TypeAlias
 from ._utils import extract_annotations, is_dataclass, is_function, is_name, is_pydantic_model
-from .errors import PYD001, PYD002, PYD003, PYD004, PYD005, PYD010, Error
+from .errors import PYD001, PYD002, PYD003, PYD004, PYD005, PYD006, PYD010, Error
 
 ClassType: TypeAlias = Literal["pydantic_model", "dataclass", "other_class"]
 
@@ -95,6 +95,17 @@ def _check_pyd_005(self, node: ast.ClassDef) -> None:
                     if previous_targets & extract_annotations(stmt.annotation):
                         self.errors.append(PYD005.from_node(stmt))
 
+    def _check_pyd_006(self, node: ast.ClassDef) -> None:
+        if self.current_class in {"pydantic_model", "dataclass"}:
+            previous_targets: set[str] = set()
+
+            for stmt in node.body:
+                if isinstance(stmt, ast.AnnAssign) and isinstance(stmt.target, ast.Name):
+                    if stmt.target.id in previous_targets:
+                        self.errors.append(PYD006.from_node(stmt))
+
+                    previous_targets.add(stmt.target.id)
+
     def _check_pyd_010(self, node: ast.ClassDef) -> None:
         if self.current_class == "other_class":
             for stmt in node.body:
@@ -115,6 +126,7 @@ def visit_ClassDef(self, node: ast.ClassDef) -> None:
         self.enter_class(node)
         self._check_pyd_002(node)
         self._check_pyd_005(node)
+        self._check_pyd_006(node)
         self._check_pyd_010(node)
         self.generic_visit(node)
         self.leave_class()

tests/rules/test_pyd006.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import ast
+
+import pytest
+
+from flake8_pydantic.errors import PYD006, Error
+from flake8_pydantic.visitor import Visitor
+
+PYD006_1 = """
+class Model(BaseModel):
+    x: int
+    x: str = "1"
+"""
+
+PYD006_2 = """
+class Model(BaseModel):
+    x: int
+    y: int
+"""
+
+
+@pytest.mark.parametrize(
+    ["source", "expected"],
+    [
+        (PYD006_1, [PYD006(4, 4)]),
+        (PYD006_2, []),
+    ],
+)
+def test_pyd006(source: str, expected: list[Error]) -> None:
+    module = ast.parse(source)
+    visitor = Visitor()
+    visitor.visit(module)
+
+    assert visitor.errors == expected