Videotollbox hardware acceleration method double displayed on macOS

[Samuel]

Summary of the bug:
I was able to reproduce it on macOS 64-bit using the latest Zeranoe build the -hwaccels display 2 times "videotoolbox" hardware acceleration method.

How to reproduce:

% ffmpeg -hwaccels
  ...
  libavutil      56. 18.100 / 56. 18.100
  libavcodec     58. 19.101 / 58. 19.101
  libavformat    58. 13.101 / 58. 13.101
  libavdevice    58.  4.100 / 58.  4.100
  libavfilter     7. 21.100 /  7. 21.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Hardware acceleration methods:
videotoolbox
videotoolbox

See the user report: ​https://github.com/UniversalMediaServer/UniversalMediaServer/issues/1592

[Samuel]

On Windows it is the "qsv" method that is displayed 2 times:

ffmpeg.exe -hwaccels
ffmpeg version N-91977-ge33b28cc79 Copyright (c) 2000-2018 the FFmpeg developers

built with gcc 8.2.0 (GCC)
configuration: --enable-gpl --enable-version3 --enable-avisynth --enable-fontconfig --enable-gnutls --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libkvazaar --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libxml2 --enable-libzimg --enable-opengl --enable-pthreads --disable-w32threads --disable-dxva2 --disable-debug --pkg-config-flags=--static --extra-ldflags=-static --extra-cflags='-mtune=generic -mfpmath=sse -msse' --cpu=i686 --optflags='-O2 -finline-fun libavutil 56. 19.101 / 56. 19.101
libavcodec 58. 30.100 / 58. 30.100
libavformat 58. 18.101 / 58. 18.101
libavdevice 58. 4.103 / 58. 4.103
libavfilter 7. 32.100 / 7. 32.100
libswscale 5. 2.100 / 5. 2.100
libswresample 3. 2.100 / 3. 2.100
libpostproc 55. 2.100 / 55. 2.100

Hardware acceleration methods:
qsv
d3d11va
qsv