How to double video frames without any duplicate frames
Use

ffmpeg -i in.avi -r 50 -filter:v "setpts=0.5*PTS" out.avi


or

ffmpeg -i in.avi -r 50 -filter:v "setpts=N/50/TB" out.avi




If you want to interpolate new frames, use

ffmpeg -i in.avi -r 50 -filter:v "yadif,framerate=50" out.avi


yadif is a deinterlacing filter, which is needed since the framerate filter does not operate on interlaced media.