Some services like gmail and outlook.com automatically save sent messages in the Sent folder. In such cases, which may or may not apply to you, it's recommended to uncheck 'Place a copy in...' in Copies & Folders settings to avoid duplicates. One would expect this to be independent of the 'Place replies in the folder...' setting, but that might not be correct. In the meantime, consider [https://addons.mozilla.org/en-US/thunderbird/addon/copy-sent-to-current/ this add-on] and see if it performs the action you desire. There are [https://bugzilla.mozilla.org/buglist.cgi?quicksearch=place%20replies%20in%20the%20folder&list_id=13561348 a few bugs] related to this setting that might offer more insight.