# V3058\. An item with the same key has already been added\.

The analyzer detected an issue that has to do with adding values to a dictionary for a key already present in this dictionary\. It will cause raising an ArgumentException at runtime with the message: "An item with the same key has already been added"\.

Consider the following example:

```cpp
var mimeTypes = new Dictionary<string, string>();
mimeTypes.Add(".aif", "audio/aiff");
mimeTypes.Add(".aif", "audio/x-aiff"); // ArgumentException
```

In this code, an ArgumentException will be raised when attempting to add a value for the "\.aif" key for the second time\.

To make this code correct, we must avoid duplicates of keys when filling the dictionary:

```cpp
var mimeTypes = new Dictionary<string, string>();
mimeTypes.Add(".aif", "audio/aiff");
```