Here's the case: I just changed the separation pad in the tray and the rubber on the feeder roller. The pages seem to be separate, but instead of stopping the feed after the end of first page, it continues as if were a roll of paper.
The pages go one after another (not overlapping) even with a single-page job.
I checked the paper setting, it's correct Letter. Not sure what should make the feeder stop pulling more pages after the first one is in.