As described in the Upgrade to Drupal 8 handbook, upgrading from Drupal 6 or Drupal 7 to Drupal 8 should be done so that the Drupal 8 site is completely empty. For example, if you manually create a node to the Drupal 8 site before upgrading and the source Drupal 6 / 7 site has a node with the same ID, the migration system will overwrite the node that was manually created in Drupal 8.
If the upgrade to Drupal 8 is done using the web browser UI (Drupal 8 path /upgrade) and conflicting IDs are detected, a warning will be shown that content may be overwritten. ID conflicts for translations are not automatically detected. For this reason, the user is instructed to refer to the upgrade handbook on how to avoid conflicts with translations. See an example screenshot below.
You can either ignore this warning and risk losing data or abort and take an alternative approach as described in the Known Issues documentation.
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