بسم الله الرحمن الرحيم
In fardh salah, it is necessary that both the imam and muqtadi are performing same salah:
قال فی الدر مع الرد 2/324،325 ط مکتبہ زکریا دیوبند: ولا مفترض بمفترض فرضا آخر لان اتحاد الصلاتین شرط عندنا اھ۔ وفی الرد: قولہ “وبمفترض فرضا آخر”: سواء تغایر الفرضان اسماً وصفۃً کمصلی ظھر امس بمصلی ظھر الیوم اھ۔
Likewise, one who performs fardh cannot follow one who is offering sunnah, the salah of muqtadi will not be valid:
الدر المختار مع الرد 2/324: ولا مفترض بمنفل اھ۔
So if you were offering any missed Maghrib salah at the time of Maghrib and someone followed you mistaking that you were offering today’s Maghrib, or you were offering sunnah salah and someone followed you thinking that you were offering fardh, their salah will not be valid, they will have to reoffer their salah. And you should also not say the takbeers loudly like an imam at such occasions and tell that person after salah that you were offering missed or sunnah salah so that they can reoffer their salah.
(2) It is makrooh tahrimi to perform second or third jamat in the mosque of the locality; those who offer such salah do not get reward of attending jamat rather the rewards decreases as it is makrooh tahrimi. At such occasions, one should do the second jamat outside the premises of the mosque or perform salah individually.
Allah knows Best!
Darul Uloom Deoband