avoid duplicate byte order marks when appending text

path/__init__.py

@@ -900,8 +900,12 @@ def write_text(
         """
         if linesep is not None:
             text = U_NEWLINE.sub(linesep, text)
-        bytes = text.encode(encoding or sys.getdefaultencoding(), errors)
-        self.write_bytes(bytes, append=append)
+        encoding = encoding or sys.getdefaultencoding()
+        if append:
+            with self.open('a', encoding=encoding, errors=errors, newline='') as f:
+                f.write(text)
+        else:
+            self.write_bytes(text.encode(encoding, errors))
 
     def lines(
         self,

tests/test_path.py

@@ -727,7 +727,7 @@ def test_patterns(self, tmpdir):
         self.assertList(d.walkfiles('*.tmp'), [e / 'x.tmp' for e in dirs])
         self.assertList(d.walkdirs('*.tmp'), [d / 'xdir.tmp'])
 
-    encodings = 'UTF-8', 'UTF-16BE', 'UTF-16LE', 'UTF-16'
+    encodings = 'UTF-8', 'UTF-16BE', 'UTF-16LE', 'UTF-16', 'UTF-32', 'UTF-8-SIG'
 
     @pytest.mark.parametrize("encoding", encodings)
     def test_unicode(self, tmpdir, encoding):
@@ -768,21 +768,14 @@ def test_unicode(self, tmpdir, encoding):
         assert p.lines(encoding) == expectedLines
         assert p.lines(encoding, retain=False) == stripped
 
-        # If this is UTF-16, that's enough.
-        # The rest of these will unfortunately fail because append=True
-        # mode causes an extra BOM to be written in the middle of the file.
-        # UTF-16 is the only encoding that has this problem.
-        if encoding == 'UTF-16':
-            return
-
         # Write Unicode to file using path.write_text().
         # This test doesn't work with a hanging line.
         cleanNoHanging = clean + '\n'
 
         p.write_text(cleanNoHanging, encoding)
         p.write_text(cleanNoHanging, encoding, append=True)
         # Check the result.
-        expectedBytes = 2 * cleanNoHanging.replace('\n', os.linesep).encode(encoding)
+        expectedBytes = (2 * cleanNoHanging.replace('\n', os.linesep)).encode(encoding)
         expectedLinesNoHanging = expectedLines[:]
         expectedLinesNoHanging[-1] += '\n'
         assert p.bytes() == expectedBytes
@@ -1398,3 +1391,14 @@ def test_ignore(self):
     def test_invalid_handler(self):
         with pytest.raises(ValueError):
             path.Handlers._resolve('raise')
+
+
+@pytest.mark.parametrize('encoding', ['UTF-16', 'UTF-32', 'UTF-8-SIG'])
+@pytest.mark.parametrize('exists', [False, True])
+def test_write_text_append_bom(tmp_path, encoding, exists):
+    target = Path(tmp_path) / 'text.txt'
+    if exists:
+        target.touch()
+    target.write_text('first', encoding=encoding, append=True)
+    target.write_text('second', encoding=encoding, append=True)
+    assert target.read_bytes() == 'firstsecond'.encode(encoding)