This is a known problem with VLC. No matter what settings you may try to adjust, VLC will never empty its playlist when you select and send one or more files to be played.

It always enqueues new files, even if it starts immediately to play them skipping the playback of existing playlist entries.

An immediate and serious problem with this functionality, is that you cannot enjoy Random Playback (Shuffle) for your new selection of files, because old entries remain in the playlist.

You can listen to your new files without the old ones reappearing, only if you disable Random Playback!

To solve this problem, download VLC Untied, a free program that lets you select one or more files or folders in Windows Explorer and load them to VLC media player instantly using a keyboard shortcut, even removing completely any files that may be already loaded and playing.