Ditto will appear as another Pokemon (Rattata, Pidgey, etc) until it is captured (including, in the Sightings).

But, if there also is a real Pidgey nearby, will the Sightings show multiple instances of Pidgey?  That is, if two Pidgeys were to appear in the Sightings, we could reasonably infer that a Ditto masquerading as a Pidgey was nearby.