What steps will reproduce the problem?

ask annex get in parallel files which point to the same key

What version of git-annex are you using? On what operating system?

6.20170815+gitg22da64d0f-1~ndall+1

Please provide any additional information below.

# works in serial mode

$> git annex get rh.white{,_avg}
get rh.white (from web...) 
/mnt/btrfs/scrap/tmp/ds0001 100%[===========================================>] 360.31K  --.-KB/s    in 0.1s    
2017-08-30 10:08:02 URL:https://dl.dropboxusercontent.com/s/0lww4tomnwfanwd/rh.white_avg?dl=0 [368962/368962] -> "/mnt/btrfs/scrap/tmp/ds000114/derivatives/freesurfer/.git/annex/tmp/MD5E-s368962--99a4db61cedffee686aef99b2d197794" [1]
(checksum...) ok
(recording state in git...)
(dev)2 10016.....................................:Wed 30 Aug 2017 10:08:02 AM EDT:.
(git)smaug:/btrfs/scrap/tmp/ds000114/derivatives/freesurfer[master]fsaverage5/surf
$> git annex drop --fast rh.white{,_avg}         
drop rh.white (checking https://dl.dropbox.com/s/0lww4tomnwfanwd/rh.white_avg?dl=0...) ok
(recording state in git...)

# "fails" in parallel
$> git annex get -J2 rh.white{,_avg}   
get rh.white get rh.white_avg (transfer already in progress, or unable to take transfer lock) 
  Unable to access these remotes: web
(from web...) 

  Try making some of these repositories available:
        00000000-0000-0000-0000-000000000001 -- web
        5e47b3f3-f09c-4969-8885-920a49ff8a45 -- yoh@smaug:/mnt/btrfs/datasets/datalad/crawl/workshops/nih-workshop-2017/ds000114/derivatives/freesurfer
failed
/mnt/btrfs/scrap/tmp/ds0001 100%[===========================================>] 360.31K  1.63MB/s    in 0.2s    
2017-08-30 10:08:21 URL:https://dl.dropboxusercontent.com/s/0lww4tomnwfanwd/rh.white_avg?dl=0 [368962/368962] -> "/mnt/btrfs/scrap/tmp/ds000114/derivatives/freesurfer/.git/annex/tmp/MD5E-s368962--99a4db61cedffee686aef99b2d197794" [1]
(checksum...) ok
(recording state in git...)
git-annex: get: 1 failed
(dev)2 10018 ->1.....................................:Wed 30 Aug 2017 10:08:21 AM EDT:.

so at the end we get a run of git-annex which exits with error 1... and in json mode also the error(s) reported etc. I wondered if annex should first analyze passed paths to get actual keys to be fetched?