Q:

Grabbing the most duplicated letter from a string

What I want to get accomplished is an algorithm that finds the most duplicated letter from the entire list of strings. I'm new to Python so its taken me roughly two hours to get to this stage. The problem with my current code is that it returns every duplicated letter, when I'm only looking for the most duplicated letter. Additionally, I would like to know of a faster way that doesn't use two for loops. 
Code:
rock_collections = ['aasdadwadasdadawwwwwwwwww', 'wasdawdasdasdAAdad', 'WaSdaWdasSwd', 'daWdAWdawd', 'QaWAWd', 'fAWAs', 'fAWDA']

seen = []
dupes = []

for words in rock_collections:
  for letter in words:
    if letter not in seen:
      seen.append(letter)
    else:
      dupes.append(letter)

print(dupes)

A:

If you are looking for the letter which appears the greatest number of times, I would recommend the following code:
def get_popular(strings):
    full = ''.join(strings)
    unique = list(set(full))
    return max(
        list(zip(unique, map(full.count, unique))), key=lambda x: x[1]
    )

rock_collections = [
    'aasdadwadasdadawwwwwwwwww',
    'wasdawdasdasdAAdad',
    'WaSdaWdasSwd',
    'daWdAWdawd',
    'QaWAWd',
    'fAWAs',
    'fAWDA'
]
print(get_popular(rock_collections))  # ('d', 19)

Let me break down the code for you:
full contains each of the strings together with without any letters between them. set(full) produces a set, meaning that it contains every unique letter only once. list(set(full)) makes this back into a list, meaning that it retains order when you iterate over the elements in the set.
map(full.count, unique) iterates over each of the unique letters and counts how many there are of them in the string. zip(unique, ...) puts those numbers with their respective letters. key=lambda x: x[1] is a way of saying, don't take the maximum value of the tuple, instead take the maximum value of the second element of the tuple (which is the number of times the letter appears). max finds the most common letter, using the aforementioned key.

