Why are post-up commands in /etc/network/interfaces ran multiple times at boot?

Files in /etc/network/if-up.d already run automatically whenever an interface (any interface) comes up. When you specify the same script to run again in an explicit post-up command, you only cause the script to run again. So my guess is this is what should happen:

• It runs once when lo comes up (with environment variable IFACE=lo) due to being located in /etc/network/if-up.d.
• It runs once when eth0 comes up (with environment variable IFACE=eth0) for the same reason.
• It runs again when eth0 comes up (with environment variable IFACE unset) because you asked for this in a post-up directive.

I'm not sure where the fourth time comes from, but anyway that's three already.

You need to either locate the script somewhere else and run it once using a post-up directive, or leave it where it is but don't mention it in a post-up directive and check the value of $IFACE so that it does nothing unless the desired interface (eth0) has come up.

As stated by Celada, /etc/network/if-up.d scripts are run for each interface. To avoid rule duplication simply add:

[ "$IFACE" = "eth0" ] || exit 0

at the top of your script. This will cause the script to exit immediately if the interface is not the desired one.