Незнам, дали правилно разбирам въпроса ти, но предполагам питаш, ако ключът вече съществува искаш да добавиш новото value към старото? Ако е това което питаш то тогава би било:

yourDictionary[outerKey][innerKey] += newValue;

Като съответно outerKey и innerKey ти са ключ на външен и ключ на вътрешен речник.

list вътре където ти се повтаря ключа нещо от сорта на Dictionary<string,Dictionary<string,List<string>>>....

за да запазиш  стойноти на повтарящ се ключ ти трябва list които да държи values  array също става но само ако знаеш предварително колко ще са list е  flexible и можеш да добавяш колкото си искаш.иначе ако искаш да ги събереш както в показал колегата

 Хмм пробвах, но нещо не се получава. 

Ако имате възможност да погледнете кода: https://pastebin.com/qU1XsFNz

На входа имам 

Morgana -> Truelock: 2

Morgana -> Truelock: 89

изхода трябва да получа 

Truelock: 91 т.е да съберя двете Value на един и същи key.

Това е условието:

1.History

Hey, it’s been a while and your friend and some of his friends from the parallel universe are coming home and they have something very interesting for you. They found that thanks to the different types of raindrops there are also different types of water. They are coming with packs full of jars holding different types of water. However, there is a small problem... they have placed all the jars in only one bag and… this bag is going to arrive a week later… They don’t like to unpack everything because they are lazy, so they send you a message in which is described which jar to whom belongs. Your job is to create a machine which unpacks the bag and separates the jars in a pile for every person. How does the machine know how to separate the jars? You must implement an algorithm which organizes the information from the message.

2.Description

You must read input lines until you receive the termination command “End”. On every input line (except the termination one) you will receive information in the following format:

“{person name} -> {liquid name}: {jars count}”

You must split the input and extract the needed information. Everything will be valid in the format described in the Input section.

You must collect all the different types of liquids for a person and sum the count of jars of every liquid for every person.

Input

The information will be in the following format:

• Person name – unique string which will not contain any of the characters that you have to split by (space, ‘-’, ‘>’, ‘:’)
• Liquid name – unique string which will not contain any of the characters that you have to split by (space, ‘-’, ‘>’, ‘:’)
• Jars count – integer number in range [0 … 100]

Output

You must print all liquids for a single person with their count of jars.

Order the people by their name (ascending) and the liquids by their count (ascending).

{Person Name} Liquids:

--- {Liquid Name}: {Count}

--- {Liquid Name}: {Count}

…

Examples: