Gate and monster appear when returning to Arkham from Other world
I think the "Gate and a monster" effect still needs to follow the basic rules on placing a gate. This means if you are at a location with an Elder Sign, the gate doesn't open, and if you are at a location with an open gate, then a monster surge happens. 

So, you are on a location with an open gate, and you have explored it. A gate opens in your location. This means a monster surge happens, and no second gate.