[c#] Argument Exception "Item with Same Key has already been added"

I keep getting an error with the following code:

Dictionary<string, string> rct3Features = new Dictionary<string, string>();
Dictionary<string, string> rct4Features = new Dictionary<string, string>();

foreach (string line in rct3Lines) 
{
    string[] items = line.Split(new String[] { " " }, 2, StringSplitOptions.None);

    rct3Features.Add(items[0], items[1]);

    ////To print out the dictionary (to see if it works)
    //foreach (KeyValuePair<string, string> item in rct3Features)
    //{
    //    Console.WriteLine(item.Key + " " + item.Value);
    //}

}

The error throws an ArgumentException saying,

"An item with the same key has already been added."

I am unsure after several Google searches how to fix this.

Later in the code I need to access the dictionary for a compare function:

Compare4To3(rct4Features, rct3Features);

public static void Compare4To3(Dictionary<string, string> dictionaryOne, Dictionary<string, string> dictionaryTwo)
{
    //foreach (string item in dictionaryOne)
    //{

    //To print out the dictionary (to see if it works)
    foreach (KeyValuePair<string, string> item in dictionaryOne)
    {
        Console.WriteLine(item.Key + " " + item.Value);
    }

        //if (dictionaryTwo.ContainsKey(dictionaryOne.Keys)
        //{
        //    Console.Write("True");
        //}
        //else
        //{
        //    Console.Write("False");
        //}
    //}
}

This function isn't completed, but I am trying to resolve this exception. What are the ways I can fix this exception error, and keep access to the dictionary for use with this function? Thank you

Clear the dictionary before adding any items to it. I don't know how a dictionary of one object affects another's during assignment but I got the error after creating another object with the same key,value pairs.

NB: If you are going to add items in a loop just make sure you clear the dictionary before entering the loop.

As others have said, you are adding the same key more than once. If this is a NOT a valid scenario, then check Jdinklage Morgoone's answer (which only saves the first value found for a key), or, consider this workaround (which only saves the last value found for a key):

// This will always overwrite the existing value if one is already stored for this key
rct3Features[items[0]] = items[1];

Otherwise, if it is valid to have multiple values for a single key, then you should consider storing your values in a List<string> for each string key.

For example:

var rct3Features = new Dictionary<string, List<string>>();
var rct4Features = new Dictionary<string, List<string>>();

foreach (string line in rct3Lines)
{
    string[] items = line.Split(new String[] { " " }, 2, StringSplitOptions.None);

    if (!rct3Features.ContainsKey(items[0]))
    {
        // No items for this key have been added, so create a new list
        // for the value with item[1] as the only item in the list
        rct3Features.Add(items[0], new List<string> { items[1] });
    }
    else
    {
        // This key already exists, so add item[1] to the existing list value
        rct3Features[items[0]].Add(items[1]);
    }
}

// To display your keys and values (testing)
foreach (KeyValuePair<string, List<string>> item in rct3Features)
{
    Console.WriteLine("The Key: {0} has values:", item.Key);
    foreach (string value in item.Value)
    {
        Console.WriteLine(" - {0}", value);
    }
}

To illustrate the problem you are having, let's look at some code...

Dictionary<string, string> test = new Dictionary<string, string>();

test.Add("Key1", "Value1");  // Works fine
test.Add("Key2", "Value2");  // Works fine
test.Add("Key1", "Value3");  // Fails because of duplicate key

The reason that a dictionary has a key/value pair is a feature so you can do this...

var myString = test["Key2"];  // myString is now Value2.

If Dictionary had 2 Key2's, it wouldn't know which one to return, so it limits you to a unique key.

If you want "insert or replace" semantics, use this syntax:

A[key] = value;     // <-- insert or replace semantics

It's more efficient and readable than calls involving "ContainsKey()" or "Remove()" prior to "Add()".

So in your case:

rct3Features[items[0]] = items[1];

That Exception is thrown if there is already a key in the dictionary when you try to add the new one.

There must be more than one line in rct3Lines with the same first word. You can't have 2 entries in the same dictionary with the same key.

You need to decide what you want to happen if the key already exists - if you want to just update the value where the key exists you can simply

rct3Features[items[0]]=items[1]

but, if not you may want to test if the key already exists with:

if(rect3Features.ContainsKey(items[0]))
{
    //Do something
} 
else 
{
    //Do something else
}