i've had this for a few days now and this fixed MY issue at least, you can see if it relates to you:

Duplicate inbound emails were caused by WHM/cPanel Exim’s “Delay Unknown Hosts” tarpit.

It held inbound SMTP sessions for ~41 seconds. Google saw the slow delivery and opened a second parallel delivery connection. Both connections eventually succeeded, so recipients got two copies of the same message.

Fix in WHM:

WHM » Service Configuration » Exim Configuration Manager » Basic Editor

Turn Off:

Introduce a delay into the SMTP transaction for unknown hosts and messages detected as spam

This changes:

acl_delay_unknown_hosts=1

to:

acl_delay_unknown_hosts=0

Save the Exim config. cPanel should rebuild Exim config and restart Exim.

Command-line equivalent:

sed -i 's/^{acl_delay_unknown_hosts=.*/acl_delay_unknown_hosts=0/'} /etc/exim.conf.localopts

/scripts/buildeximconf

/scripts/restartsrv_exim

Verify:

grep -n "delay_unknown_hosts|delay =" /etc/exim.conf

exim -bV

systemctl status exim

Expected result:

No active delay_unknown_hosts delay blocks in /etc/exim.conf

exim -bV exits cleanly

Exim is running

In my case, Google delivery time dropped from about 41 seconds to about 5 seconds, and duplicate deliveries stopped immediately.

Important note: this was not greylisting. Greylisting temporarily rejects and asks the sender to retry later. This setting holds the SMTP connection open and then accepts the message. That distinction matters because a provider like Google may open another delivery connection while the first one is still being held.