Saving changes to Python files with 'Format On Save' occasionally leads to duplicating the last few lines or symbols (using Black Formatter)

[kirilllzaitsev]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version:

Version: 1.107.1
Commit: 994fd12f8d3a5aa16f17d42c041e5809167e845a
Date: 2025-12-17T14:15:14.850Z
Electron: 39.2.3
ElectronBuildId: 12895514
Chromium: 142.0.7444.175
Node.js: 22.21.1
V8: 14.2.231.21-electron.0
OS: Linux x64 6.8.0-90-generic

• OS Version: Ubuntu 22.04

Example. The last two lines on the screenshot were duplicated twice over multiple Save+Format on save executions:

Steps to Reproduce:

1. Create a Python file with some code
2. Save with format-on-save (e.g., Black) multiple times (might require some violation of the PEP8 style to take effect)
3. Observe new statements (valid or invalid) appear at the end of the file

The issue has been there for a few months.

[eleanorjboyd]

Hi! Does this only occur with the black formatter or any formatter you select?

[kirilllzaitsev]

Hi. No issues after a day of using autopep8.

[kirilllzaitsev]

A few more days of autopep8 usage, and the issue repeats there as well

[Quentin62]

Same issue. I think it is related to isort

I have the following settings:

    "[python]": {
        "editor.defaultFormatter": "ms-python.black-formatter",
        "editor.formatOnSave": true,
        "editor.codeActionsOnSave": {
            "source.organizeImports": "explicit"
        },
    },
    "isort.args": [
        "--profile",
        "black",
        "-l 127"
    ],
    "black-formatter.args": [
        "--line-length=127"
    ],

when I add an import statement in the wrong position and save to format, i get a duplicated line

Enregistrement.de.l.ecran.2026-01-14.122716.mp4