In Unity3D I am reading a textfile with information on nodes and its connections.
However it seems that ALL duplicates are removed automatically!
Even though split shouldn't remove duplicates (tested here) you could try using Regex and Linq.
var words = Regex.Matches("Node,Node2,Node2,Lamp,Lamp,Node1,Node3,Node4", @",?(\w+)").Cast().Select(x => x.Groups[1].Value);