why is this marked as fixed?

Still a serious problem in 5.3.3f1

Is that right; this won't be fixed till 5.4?

Ok, it did NOT fix the problem for me. If I join a lobby of a NetworkLobbyManager, then disconnect from the lobby and try to reconnect to the lobby, the same error occurs again.

Unity 5.2.2f1 fixed the problem for me.

5.2.2p1 still nothing. When will it be fixed?

5.2.2! Future release is 5.3 or what?

ArgumentException: An element with the same key already exists in the dictionary.
System.Collections.Generic.Dictionary`2[UnityEngine.Networking.Types.NetworkID,UnityEngine.Networking.Types.NetworkAccessToken].Add (NetworkID key, UnityEngine.Networking.Types.NetworkAccessToken value)

Still having this issue on 5.2.1 !

In Unity 5.2.1

ArgumentException: An element with the same key already exists in the dictionary.
System.Collections.Generic.Dictionary`2[UnityEngine.Networking.Types.NetworkID,UnityEngine.Networking.Types.NetworkAccessToken].Add (NetworkID key, UnityEngine.Networking.Types.NetworkAccessToken value) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/List.cs:101)
UnityEngine.Networking.Utility.SetAccessTokenForNetwork (NetworkID netId, UnityEngine.Networking.Types.NetworkAccessToken accessToken) (at C:/buildslave/unity/build/Runtime/Networking/Managed/UNETTypes.cs:119)
UnityEngine.Networking.NetworkManager.OnMatchJoined (UnityEngine.Networking.Match.JoinMatchResponse matchInfo) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:1022)
UnityEngine.Networking.Match.NetworkMatch+<ProcessMatchResponse>c__Iterator0`1[UnityEngine.Networking.Match.JoinMatchResponse].MoveNext () (at C:/buildslave/unity/build/Runtime/Networking/Managed/MatchMakingClient.cs:302)

Until this bughas been officially resolved. Hook your own callback to the JoinMatch method.

this.manager.matchMaker.JoinMatch(networkID, string.Empty, new NetworkMatch.ResponseDelegate<JoinMatchResponse>(OnMatchJoined));

public virtual void MatchJoined(JoinMatchResponse matchInfo)
{
if (LogFilter.logDebug)
{
Debug.Log("NetworkManager OnMatchJoined ");
}
if (matchInfo.success)
{
try
{
Utility.SetAccessTokenForNetwork(matchInfo.networkId, new NetworkAccessToken(matchInfo.accessTokenString));
}
catch(Exception ex)
{
if (LogFilter.logError)
{
Debug.LogError(ex);
}
}
this.StartClient(new MatchInfo(matchInfo));
}
else if (LogFilter.logError)
{
Debug.LogError(string.Concat("Join Failed:", matchInfo));
}
}

This solved the problem for us.

I have the same Issue. Unity this is a must fix issue. I cant develop my game any further without it fixed.

To recreate download and run the NetworkStarter demo from below:
http://forum.unity3d.com/threads/unet-sample-projects.331978/

"ArgumentException: An element with the same key already exists in the dictionary.
System.Collections.Generic.Dictionary`2[UnityEngine.Networking.Types.NetworkID,UnityEngine.Networking.Types.NetworkAccessToken].Add (NetworkID key, UnityEngine.Networking.Types.NetworkAccessToken value) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:404)
UnityEngine.Networking.Utility.SetAccessTokenForNetwork (NetworkID netId, UnityEngine.Networking.Types.NetworkAccessToken accessToken) (at C:/buildslave/unity/build/Runtime/Networking/Managed/UNETTypes.cs:119)
UnityEngine.Networking.NetworkManager.OnMatchJoined (UnityEngine.Networking.Match.JoinMatchResponse matchInfo) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:1010)
UnityEngine.Networking.Match.NetworkMatch+<ProcessMatchResponse>c__Iterator0`1[UnityEngine.Networking.Match.JoinMatchResponse].MoveNext () (at C:/buildslave/unity/build/Runtime/Networking/Managed/MatchMakingClient.cs:302)"