[FFmpeg-user] Why are half the frames "duplicate" ?

[sean darcy]

I'm transcoding mpeg2 -> x264.

Stream #0:0[0x64]: Video: mpeg2video (Main) ([2][0][0][0] / 0x0002), 
yuv420p(tv, progressive), 720x480 [SAR 8:9 DAR 4:3], Closed Captions, 
29.97 fps, 59.94 tbr, 90k tbn, 59.94 tbc

When I run ffmpeg:

ffmpeg -i in.mpg -map 0:0 -map 0:2 -c:v libx264 -tune film -preset slow 
-crf 22 -c:a copy out.mp4

almost half the frames are dups:

frame=394550 fps=135 q=-1.0 Lsize= 2218652kB time=03:39:24.71 
bitrate=1380.6kbits/s dup=175142 drop=0 speed=4.51x

Does this really mean that there were 175,142 frames with exact copies ? 
So only ( 394,550 less 175,142 ) unique frames ?

The output looks fine. The input looks fine. What are these duplicated 
frames? Is it an interlacing issue ? But the input is progressive.

Does it have to do with this warning:

[mp4 @ 0x921c80] track 1: codec frame size is not set
Output #0, mp4, to 'out.mp4':
   Metadata:
     encoder         : Lavf58.18.103
     Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 
720x480 [SAR 8:9 DAR 4:3], q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc
     Metadata:
       encoder         : Lavc58.31.100 libx264

Puzzled.