Here is the code with the mean, median and mode functions implemented and fully rewritten:

from collections import Counter

def calculate_mean(numbers):
    # Calculate the mean by summing all numbers and dividing by the total count
    return sum(numbers) / len(numbers)

def calculate_median(numbers):
    # Sort the numbers and retrieve the middle value(s) based on whether the list has an odd or even length
    sorted_numbers = sorted(numbers)
    length = len(numbers)
    middle_index = length // 2
    
    if length % 2 == 0:
        return (sorted_numbers[middle_index - 1] + sorted_numbers[middle_index]) / 2
    else:
        return sorted_numbers[middle_index]

def calculate_mode(numbers):
    # Retrieve the most common number(s) using Counter
    count = Counter(numbers)
    max_count = max(count.values())
    mode = [k for k, v in count.items() if v == max_count]
    
    if len(mode) == len(numbers):
        return None
    else:
        return mode

# Test the functions
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 9]
print("Mean:", calculate_mean(numbers))
print("Median:", calculate_median(numbers))
print("Mode:", calculate_mode(numbers))

The output for the test case should be:

Mean: 5.5
Median: 6.0
Mode: [9]
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