use strict;
use warnings;
use Data::Dump::Streamer;

my $allusers = [ {...} ]; # Data per sample
my %users;

for my $user (@$allusers) {
    my $name = $user->{user};

    $users{$name}{$_} += $user->{$_} for qw(duration amount);
}

@$allusers = map {
    {user => $_, duration => $users{$_}{duration}, amount => $users{$_}{amount}}
    } sort keys %users;

Dump $allusers;