collectinfo.pl running for long time

A few times a day, I get an email from the cron daemon on my Virtualmin server:

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From: root@ranch.binarytribe.com (Cron Daemon)
To: root@ranch.binarytribe.com
Subject: Cron /etc/webmin/virtual-server/collectinfo.pl

Already running
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This started a few months ago, and I used to get dozens of these a day until I modified Virtualmin's config so that the collectinfo.pl script only ran every half hour instead of every 5 minutes. But even with the 30 minute spread it still occurs, so I'm wondering what's wrong with my system that's causing the script to take so long to run that it overlaps itself? It's a reasonably fast server with plenty of resources and only ~10 domains. What's the best way to debug what's causing the delay?