Originally was trying to reproduce datalad/issues/3653 assuming that multiple files pointed to the same key. It was not the case, and my attempt revealed another bug - annex inability to "obtain" files in parallel when multiple of them point to the same key:

setup of original repo(click to expand)

/tmp > mkdir src; (cd src; git init; git annex init; dd if=/dev/zero of=1 count=1024 bs=1024; for f in {2..10}; do cp 1 $f; done ; git annex add *; git commit -m added; )
Initialized empty Git repository in /tmp/src/.git/
init  (scanning for unlocked files...)
(recording state in git...)
1024+0 records in
1024+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00106651 s, 983 MB/s
add 1 
add 10 
add 2 
add 3 
add 4 
add 5 
add 6 
add 7 
add 8 
add 9 
(recording state in git...)
[master (root-commit) 63b1163] added
 10 files changed, 10 insertions(+)
 create mode 120000 1
 create mode 120000 10
 create mode 120000 2
 create mode 120000 3
 create mode 120000 4
 create mode 120000 5
 create mode 120000 6
 create mode 120000 7
 create mode 120000 8
 create mode 120000 9

And that is what happens then when we try to get the same key in parallel:

/tmp > git clone src dst; (cd dst; git annex get -J 5 *; )
Cloning into 'dst'...
(merging origin/git-annex into git-annex...)
(recording state in git...)
(scanning for unlocked files...)
get 2 (from origin...) (checksum...) 
git-annex: thread blocked indefinitely in an STM transaction
git-annex: thread blocked indefinitely in an MVar operation

I felt like it is an old issue but failed to find a trace of it upon a quick lookup

fixed --Joey