[perl #122714] &done on Supply from Proc::Async called more than once

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# New Ticket Created by  Leon Timmermans 
# Please include the string:  [perl #122714]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=122714 >


The done handler on a on a supply can be called more than once, this
shouldn't happen.
Take for example:

my $proc = Proc::Async.new(:path("echo"), :args([<Hello World>]));
my $in = $proc.stdout_chars(); $in.tap(-> $data { }, :done({ say "Done
called"}));
await $proc.start;'

This will output «Done called» twice.

Leon