If you open your mouth with a sentence you and hear someone sharing in sync with you kind of like puppet that is tall tail sign. Don't say anything a complete whisper it is behind you with you speaking especially if you recognize it as someone else you once or currently know but is you because there is no current squabble don't mean there won't be . As if you are behind a glass and the external person can't hear do this with yourself the previous sentence whisper it out see if an introject is doing the talking. I didn't make this discovery until some years later so I don't know if this good for a completely dissociated beginner starting out in therapy.