[c#] Duplicate keys in .NET dictionaries?

I stumbled across this post in search of the same answer, and found none, so I rigged up a bare-bones example solution using a list of dictionaries, overriding the [] operator to add a new dictionary to the list when all others have a given key(set), and return a list of values (get).
It's ugly and inefficient, it ONLY gets/sets by key, and it always returns a list, but it works:

 class DKD {
        List<Dictionary<string, string>> dictionaries;
        public DKD(){
            dictionaries = new List<Dictionary<string, string>>();}
        public object this[string key]{
             get{
                string temp;
                List<string> valueList = new List<string>();
                for (int i = 0; i < dictionaries.Count; i++){
                    dictionaries[i].TryGetValue(key, out temp);
                    if (temp == key){
                        valueList.Add(temp);}}
                return valueList;}
            set{
                for (int i = 0; i < dictionaries.Count; i++){
                    if (dictionaries[i].ContainsKey(key)){
                        continue;}
                    else{
                        dictionaries[i].Add(key,(string) value);
                        return;}}
                dictionaries.Add(new Dictionary<string, string>());
                dictionaries.Last()[key] =(string)value;
            }
        }
    }

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