libavcodec_x86_x86asm_optional_sources = {
  ...
  'adpcm_g722_decoder' : files(['g722dsp.asm',]),
  'adpcm_g722_encoder' : files(['g722dsp.asm',]),
  ...
 }
later:

asm_gen = generator(x86asm,
  output : '@BASENAME@.o',
  arguments : [
    '-f', asm_format,
    '-i', asm_inc,
    '@INPUT@',
    '-o', '@OUTPUT@'] + asm_args)

later:

both components are enabled, x86asm_sources thus contains [..., 'g722dsp.asm', 'g722dsp.asm', ...]

sources += asm_gen.process(x86asm_sources)

Boom:

Multiple producers for Ninja target "avcodec@sha/g722dsp.o". Please rename your targets.