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to understand what it refers to when it states keys:

 http://msdn.microsoft.com/en-us/library/ms132330(v=vs.110).aspx

you can only have one type of key (no duplicates) but you can have multiple duplicate values!

http://msdn.microsoft.com/en-us/library/ms132379(v=vs.110).aspx

the solution is from here:

http://community.office365.com/en-us/forums/154/t/184781.aspx

download sharepoint manager 2013:

http://spm.codeplex.com/

to understand what it refers to when it states keys:  SortedList < TKey, TValue >.Add

You can only have one type of key (no duplicates) but you can have multiple duplicate values! : SortedList < TKey, TValue >.Keys

the solution is from here.

download sharepoint manager 2013.

EDIT

somthing is wrong with your sever to bring that error :s , but did find a solution to your origonal question tho!

the solution is from here:

http://community.office365.com/en-us/forums/154/t/184781.aspx

Solution:

download sharepoint manager 2013:

http://spm.codeplex.com/

goto parent site then look for Fields and deleted the HashTags field.

EDIT

forgot to add console.read!

   static void Main(string[] args)
    {

        using (SPSite mysite = new SPSite("http://yourTestSite"))
        {

            using (SPWeb web = mysite.OpenWeb())
            {

                SPListCollection collList = web.Lists;

                foreach (SPList list in collList)
                {
                    foreach (SPListItem item in list.Items)
                    {
                        foreach (SPField field in item.Fields)
                        {
                            if (field.TypeAsString != "HashTags")
                            {
                                var listlocal = list.DefaultDisplayFormUrl;
                                var listName = list.Title;
                                var fld = field.Title;
                                
                                    Console.Out.Write("List Url =" + listlocal);
                                    Console.Out.Write("List name =" + listName);
                                    Console.Out.Write("Field name =" + fld);
                                    Console.Out.WriteLine();
                            }
                        }
                    }
                }
            }

        }
        Console.Read();
    }