Migrate file not replacing files having same file name

You can just use file_exists: rename for this.

- file_exists: (optional) Replace behavior when the destination file already
   exists:
   - 'replace' - (default) Replace the existing file.
   - 'rename' - Append _{incrementing number} until the filename is
       unique.
   - 'use existing' - Do nothing and return FALSE.

Example:

  field_images:
    plugin: file_import
    source: images
    destination: constants/image_destination
    skip_on_missing_source: true
    file_exists: rename

I can confirm that file_exists: replace works as well. Images with the same filenames are replaced correctly, but the old image styles were not flushed. See issue #3420449 🐛 Flush image styles when replace is used Needs review