diff --git a/plugins/py_detect.py b/plugins/py_detect.py
index e0ccf6e..7b3c08d 100644
--- a/plugins/py_detect.py
+++ b/plugins/py_detect.py
@@ -686,9 +686,8 @@ def analyze(source: str, filename: str = "<string>") -> List[Dict]:
                               extra=f"重复定义: {name}",
                               name=name, first_line=nodes[0].lineno)
 
-    issues.extend(analyzer.issues)
     _dataflow_check(tree, analyzer)                     # L3: 数据流 (部分路径未定义)
-    issues.extend(analyzer.issues)
+    issues.extend(analyzer.issues)                      # L1+L3 只 extend 一次, 避免重复
     issues = _jedi_validate(source, filename, issues)   # L2: 语义二次确认 (可选)
     return issues
 
diff --git a/tests/test_py_detect.py b/tests/test_py_detect.py
index 1215869..b930d47 100644
--- a/tests/test_py_detect.py
+++ b/tests/test_py_detect.py
@@ -366,6 +366,18 @@ class TestDataFlow:
                 fix = d.get("fix", "")
         assert "result = None" in fix, fix
 
+    def test_no_duplicate_diagnostics(self, pd):
+        """L1+L3 合并不得产生重复诊断 (回归: 曾因 extend 两次全部重复)"""
+        src = ("import os\n"
+               "from math import *\n"
+               "def f(flag):\n"
+               "    if flag:\n"
+               "        r = build()\n"
+               "    return r\n")
+        issues = pd.analyze(src, "t.py")
+        keys = [(d["code"], d["line"], d["col"]) for d in issues]
+        assert len(keys) == len(set(keys)), f"存在重复诊断: {keys}"
+
 
 # ── code_editor 写文件后自动检测集成 ──
 class TestAutoDetectIntegration: