When uploading synonyms via a CSV file, existing synonyms are only overwritten if both of the following are true:
The existing synonym was originally created via the Dashboard
The content of the new synonym is exactly identical to the existing one
Each synonym entry in the Algolia index has an objectID value that works as a unique identifier. When uploading synonyms via the API or in JSON format, you can explicitly define the objectID. When uploading in CSV format via the Dashboard however, you cannot specify an objectID - one will be generated automatically.
In that case, the Dashboard generates an objectID based the synonym's content. Two synonyms will only be considered the same (and therefore overwritten) if their content matches exactly.
If you want control over which synonyms are overwritten, you should upload synonyms in JSON format or via the API so you can specify objectIDs, ensuring that existing synonyms are correctly identified and replaced, regardless of changes in their content.
For a full list of common synonym issues, troubleshooting topics and best practices, see our summary article here.