Filename always includes ;1 at the end, why?

I have some C# code that goes something like this:

using DiscUtils;
using DiscUtils.Common;
using DiscUtils.Iso9660;

public void PopulateUSBDrive(string isoSource, string volumeName) {
 using (FileStream isoStream = File.Open(@isoSource, FileMode.Open)) {
	CDReader iso = new CDReader(isoStream, true);
	CopyDirectoryTree(iso.Root, ref iso, volumeName);
	isoStream.Close();
 }
}
/// <summary>
/// Copies all files and folders from iso
/// </summary>
/// <param name="rootDir">Points to the current DiscDirectoryInfo that is the root of file copy operations</param>
/// <param name="iso">The CDReader object that represents the iso we are copying from</param>
/// <param name="destinationVolumeName">The volume name of the USB partion we are copying the iso to</param>
/// <returns></returns>
private List<DiscDirectoryInfo> CopyDirectoryTree(DiscDirectoryInfo rootDir, ref CDReader iso, string destinationVolumeName) {
 UsbDisk installDisk = usbDisks.Where(x => x.Volume == destinationVolumeName).FirstOrDefault<UsbDisk>();
 string installPartitionDriveLetter = installDisk != null ? installDisk.DiskLetter : String.Empty;

 if (!String.IsNullOrEmpty(installPartitionDriveLetter)) {
	string directory = installPartitionDriveLetter + "\\" + rootDir.FullName;
	if (!Directory.Exists(directory)) Directory.CreateDirectory(directory);
	foreach (var file in rootDir.GetFiles()) {
	   Stream fileStream = iso.OpenFile(file.FullName, FileMode.Open, FileAccess.Read);
	   using (FileStream output = new FileStream(installPartitionDriveLetter + (file.FullName.Contains(';') ? file.FullName.Split(';')[0] : file.FullName), FileMode.Create)) {                  
		  fileStream.CopyTo(output);
	   }
	}
	foreach (DiscDirectoryInfo subdir in rootDir.GetDirectories()) {
	   CopyDirectoryTree(subdir, ref iso, destinationVolumeName);
	}
 }        
 return rootDir.GetDirectories().ToList<DiscDirectoryInfo>();
}

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file.FullName and file.Name will always end with ;1 for some reason, while the true filename does not end with ;1. To write to the correct file name, I have to split the string that represents the name at ; and take the first part as the valid file name(see line 28). Why does this happen, am I doing something wrong? Any way to avoid this?