import re

pattern = r"([ag])\1+|([^\sag])\2+"
s = "mengganti, maaf, ppuuutttiiiihhh, mmmmeeeeerrrraaaah, maaagggz"
result = re.sub(
        pattern,
        lambda x: x.group(1) * 2 if x.group(1) else x.group(2),
        s
)

if result:
        print(result)