ffmpeg deinterlace

You'll probably find the the yadif section of the FFMPEG documentation pretty interesting:

Deinterlace the input video ("yadif" means "yet another deinterlacing filter"). 
It accepts the optional parameters: mode:parity:auto. 

MODE specifies the interlacing mode to adopt, accepts one of the following values: 
‘0’
output 1 frame for each frame
‘1’
output 1 frame for each field 
‘2’
like 0 but skips spatial interlacing check 
‘3’
like 1 but skips spatial interlacing check 
Default value is 0. 

PARITY default value is -1.
AUTO default value is 0. 

Notice how the default MODE is 0, which instructs FFMPEG to ouput 1 frame for each 2 fields, thus halving the fps. I think what you're looking for isMODE 1: one frame per field. I'm not too sure how to use these optional parameters but perhaps something like

-filter:v yadif 1:-1:0 -s "1280x720" -r 50