Duplicate Cleaner uses the following great libraries:

SQLite / System.Data.SQLite 
Public Domain
https://system.data.sqlite.org

ObjectListView 2.9 by Philip Piper / BrightIdeas Software
Used under a commercial license.
http://objectlistview.sourceforge.net/cs/index.html

TagLib# 2.1
The GNU Lesser General Public License, version 2.1 (LGPL-2.1)
https://github.com/mono/taglib-sharp

DotNetZip (Ionic's Zip Library) 1.9.1.8 
(c)Dino Chiesa 2006-2011
Microsoft Public License (Ms-PL)
https://dotnetzip.codeplex.com/

FreeImage 3.17
FreeImage Public License - Version 1.0
http://freeimage.sourceforge.net

NAudio 1.8
Microsoft Public License (Ms-PL)
https://github.com/naudio/NAudio

AcoustID.NET
The GNU Lesser General Public License, version 2.1 (LGPL-2.1)
https://github.com/wo80/AcoustID.NET

Icons
The Duplicate Cleaner upgrade page uses icons by CreativeTail under CC BY 4.0 license
https://www.creativetail.com
