test.py

from itertools import zip_longest

def convert(st):
    pairs = zip_longest(st, st[1:])
    for c, n in pairs:
        if c == "e" and n != "e":
            yield "va"
        elif c == "e" and n == "e":
            yield c
            yield c
            next(pairs)
        else:
            yield c

print("".join(convert("hello ee blah this is a test weekly wee")))