C# Rename File List - Creates a New List of Files with Duplicates Renamed

RenameFileList.cs

public static class RenameFileList
    {
        /// <summary>
        ///     Creates a new list of files with duplicates renamed.
        /// </summary>
        public static IEnumerable<SourceTarget> ProcessDuplicateNames(this IEnumerable<string> lst, string targetDirectory)
        {
            var NewFileList = new CHashSet<SourceTarget>();
            var tfl         = new CHashSet<string>();
            foreach (var s in lst)
            {
                var counter   = 0;
                var directory = Path.GetDirectoryName(s);
                if (directory == null)
                    continue;
                var extension       = Path.GetExtension(s);
                var filename        = Path.GetFileNameWithoutExtension(s);
                var possibleNewName = $"{filename}{extension}";
                var st              = new SourceTarget();
                st.Source   = s;
                st.fileData = new FileData(s);
                if (!tfl.Contains(possibleNewName))
                {
                    tfl.Add(possibleNewName);
                    st.Target = Path.Combine(targetDirectory, $"{filename}{extension}");
                    NewFileList.Add(st);
                }
                else
                {
                    var newtargetName = "";
                    do
                    {
                        counter++;
                        possibleNewName = $"{filename}_Copy({counter}){extension}";
                        newtargetName   = Path.Combine(targetDirectory, $"{filename}_Copy({counter}){extension}");
                    } while (tfl.Contains(possibleNewName));

                    tfl.Add(possibleNewName);
                    st.Target = newtargetName;
                    NewFileList.Add(st);
                }
            }

            return NewFileList;
        }
        public static IEnumerable<SourceTarget> ProcessDuplicateNames(this IEnumerable<FileData> lst, string targetDirectory)
        {
            var NewFileList = new CHashSet<SourceTarget>();
            var tfl         = new CHashSet<string>();
            foreach (var s in lst)
            {
                var counter   = 0;
                var directory = Path.GetDirectoryName(s.FullName);
                if (directory == null)
                    continue;
                var extension       = Path.GetExtension(s.FullName);
                var filename        = Path.GetFileNameWithoutExtension(s.FullName);
                var possibleNewName = $"{filename}{extension}";
                var st              = new SourceTarget();
                st.Source   = s.FullName;
                st.fileData = s;
                if (!tfl.Contains(possibleNewName))
                {
                    tfl.Add(possibleNewName);
                    st.Target = Path.Combine(targetDirectory, $"{filename}{extension}");
                    NewFileList.Add(st);
                }
                else
                {
                    var newtargetName = "";
                    do
                    {
                        counter++;
                        possibleNewName = $"{filename}_Copy({counter}){extension}";
                        newtargetName   = Path.Combine(targetDirectory, $"{filename}_Copy({counter}){extension}");
                    } while (tfl.Contains(possibleNewName));

                    tfl.Add(possibleNewName);
                    st.Target = newtargetName;
                    NewFileList.Add(st);
                }
            }

            return NewFileList;
        }
        public static IEnumerable<SourceTarget> ProcessDuplicateNames(this IEnumerable<FileInfo> lst, string targetDirectory)
        {
            var NewFileList = new CHashSet<SourceTarget>();
            var tfl         = new CHashSet<string>();
            foreach (var s in lst)
            {
                var counter   = 0;
                var directory = Path.GetDirectoryName(s.FullName);
                if (directory == null)
                    continue;
                var extension       = Path.GetExtension(s.FullName);
                var filename        = Path.GetFileNameWithoutExtension(s.FullName);
                var possibleNewName = $"{filename}{extension}";
                var st              = new SourceTarget();
                st.Source   = s.FullName;
                st.fileInfo = s;
                if (!tfl.Contains(possibleNewName))
                {
                    tfl.Add(possibleNewName);
                    st.Target = Path.Combine(targetDirectory, $"{filename}{extension}");
                    NewFileList.Add(st);
                }
                else
                {
                    var newtargetName = "";
                    do
                    {
                        counter++;
                        possibleNewName = $"{filename}_Copy({counter}){extension}";
                        newtargetName   = Path.Combine(targetDirectory, $"{filename}_Copy({counter}){extension}");
                    } while (tfl.Contains(possibleNewName));

                    tfl.Add(possibleNewName);
                    st.Target = newtargetName;
                    NewFileList.Add(st);
                }
            }

            return NewFileList;
        }
        public class SourceTarget
        {
            public FileData fileData;
            public FileInfo fileInfo;
            public string   Source = "";
            public string   Target = "";
        }
    }