Monica's Doubt only "mirror-spoke", as far as I can tell, when she was trying to make Monica doubt herself on an existential level (for the merely annoying,
she kept it to normal-speak). And she did not always stick hard and fast to that "rule", either:
This is
clearly Monica's Doubt, yet
when she speaks two pages later, she does not mirror-speak.
Even
just before they fix the calendar machine, she does not mirror-speak, even though she is trying to get Monica angry (in order to make her light up her face).
Look up the other links I posted before, and you will still recognize her, visually speaking, even when she is not mirror-speaking.
For instance, look again at the links I posted of the Cerberus Club, where you see both Doubts together, and you will see that the "Queen of sort" (Jin's Doubt) who wanted Monica's doorway for herself has a shorter, wider face, with distinct markings. Every time we see her, she has those markings, even when she talks to Monica that one time. The only time she did not have those markings is when Jin poited her away (which has been linked several times, by now), but she obviously came back... The other links I have provided are clearly not her (unless she is there WITH Monica's Doubt.) Jin's Doubt has only directly interacted with Monica twice: once, when she told Monica that it was Tina's demons who chased her, not her own (said interaction ended up with Monica popping her, bubble-like, with a sharp pencil), and during the calendar fix, when she tried to prevent Shelly from stabbing Monica. Any other time we see Monica interacting with demons, they are her own demons, including her own Doubt. (Except, of course, when she had to deal with Georgette's demons, but there was no confusion there. We knew they were not Monica's.)
)
My 
*cough*