Only one instance being printed when there are many others

Question

When trying to append comment to x, only one instance of comment is being printed despite there being many others.

    with open("testingusercomments.txt", "r") as a, open("testingusercommentstmp.txt", "a") as x:
        try:
            for comment in r.redditor("username").comments.new(limit=None):
                if comment not in a.read().split("\n"):
                    print(comment)
                    x.append(comment)

Although this code only returns one instance of comment the code below returns the proper amount.

with open("testingusercomments.txt", "r") as a, open("testingusercommentstmp.txt", "a") as x:
    try:
        for comment in r.redditor("username").comments.new(limit=None):
            if comment not in a.read().split("\n"):
                print(comment)

Is there a problem appending to a file while searching for comments? Is there something I've missed to solve this?

Answer 1

Although I'm unsure of the cause, I've found a solution.

x = []
for comment in r.redditor("username").comments.new(limit=None):
    if comment not in x:
        x.append(comment)