Temporary queue to prevent duplication
collections.deque

Why not use a collections.deque with a max_len?

from collections import deque

for submission in reddit.subreddit("funny").stream.submissions():
    queue = deque(max_len=10)

    if submission.id not in queue:
        if "foo" in title:
            submission.reply("Bar!")
            queue.append(submission.id)