qvm-open-in-vm doesn't work if opener already running

[airelemental]

Qubes OS version:

R4.0

Affected component(s):

qvm-open-in-vm

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Steps to reproduce the behavior:

1. In vm1, assign e.g. firefox to open files of type .png
2. In vm2, run qvm-open-in-vm vm1 a.png
3. In vm2, in another terminal, run qvm-open-in-vm vm1 b.png

Expected behavior:

a.png opens in vm1 firefox. qvm-open-in-vm blocks until a.png closed.
b.png opens in vm1 firefox. qvm-open-in-vm blocks until b.png closed.

Actual behavior:

a.png opens in vm1 firefox. qvm-open-in-vm blocks until a.png closed.
b.png sometimes opens, sometimes fails to open. qvm-open-in-vm does not block.

General notes:

Same issue happens with eog (Eye of Gnome), which also opens several documents in one process (according to top)

Issue does not happen with evince, which creates one new process per document opened.