lst = [("a", 1), ("b", 2), ("a", 3), ("b", 1), ("a", 2), ("c", 1)]

dictionary = {}

for data in lst:
    key = data[0]
    value = data[1]
    if key in dictionary.keys():
        dictionary[key].append(value)
    else:
        dictionary[key] = []
        dictionary[key].append(value)

print(dictionary)