Hey @Yi-tzu this wouldn’t currently be possible. Unique property values can only be enabled during the creation of a property, it can’t be enabled after a property has been created. Since deal name is a default value created by HubSpot, this can’t be retroactively created.
You could create a custom property for something like “unique deal key” or name it anyting you like, and have a duplicate column of the deal name and map this during the import too to enforce it being unique.