First non-repeating character in a stream

Characters arrive one at a time. After each arrival, report the first character seen so far that has appeared exactly once, or "#" if there is none. Keep a count of each character in a dictionary, and a queue of characters in arrival order. After each arrival, drop characters from the front of the queue while their count is more than one; the front, if any, is the answer.

Write first_unique_stream(text) that returns a string with one answer character per arrival.

first_unique_stream("aabc")  ->  "a#bb"
first_unique_stream("abab")  ->  "aab#"

from collections import deque

def first_unique_stream(text):
    ...