Star Trek Continues: ffmeg dvdsub: overlapping subtitles
Technically the cleanest look with these subtitles would be to:


Extract the subtitles
Turn them to text using your favourite OCR application
Carefully review and edit the text
Burn them back into the media stream in the size, colour, orientation and format of your choice


This is a large and time consuming job which I have personally undertaken in the past but is probably overkill in this case.

The better option is to simply add in the FFmpeg option -fix_sub_duration (placed before the input file options) and this will fix the doubled up subtitles as I illustrate below, and make the subtitles easily readable:



The details of the -fix_sub_duration option can be found in the FFmpeg-all man pages but I have taken the liberty of quoting these details here:


  -fix_sub_duration
  
  Fix subtitles durations. For each subtitle, wait for the next packet in the same stream and adjust the duration of the
  first to avoid overlap. This is necessary with some subtitles codecs,
  especially DVB subtitles, because the duration in the original packet
  is only a rough estimate and the end is actually marked by an empty
  subtitle frame. Failing to use this option when necessary can result
  in exaggerated durations or muxing failures due to non-monotonic
  timestamps.
  
  Note that this option will delay the output of all data until the next
  subtitle packet is decoded: it may increase memory consumption and
  latency a lot.


You will note that this is the first of the Advanced Subtitle options, the second being the -canvas_size that you have already used...