This is a common question about Duplicate Files.
When you scan one or more drives for duplicate files, it lists all the duplicate files found on the given drive(s). This does not necessarily means you can delete them, sometimes it is normal to have duplicate files (for example the Windows sysbckup Directory). It's up to you to decide which ones are needed and which ones can be deleted/moved.
If the file has the extension .dll but it is not located in the Windows System folder you should move it to the Windows System folder. You should then be able to safely delete all other files with the same name. Most of all, make sure you back up before deleting duplicate files!
One example can be the vbrun300.dll (Visual Basic 3 runtime libraries). If you have one or more copies of this file on directories besides the Windows/System one, you can delete them (and KEEP the one on the System Directory).
You need to Login as the Administrator in order to use this utility.