Show us how you have the broker node configured please. Also stop node red and start it again in a terminal and post the full startup output here, up to the point where a repeated debug output is seen.
There is a technical reason under the hood that I try to explain below however, if you have a v5 capable broker, switching to v5 will avoid this issue.
In as short a terms I can, the MQTT client provides single stream of values for topics subscribed to. There is no other associative info provided with these, so when you subscribe to abc/# and abc/def there are 2 packets sent. The MQTT config receives these 2 packets & send them to the 2 nodes.
With MQTT v5, we can add additional info to determine which packet is for which subscription & thus avoid the duplicate messages.
I confirm I can repro the issue with my broker, every debug node see the message twice --see below.
And again the broker is not the issue as all the other clients I've tested do not see message duplication (see below my repro).
If you need data I can collect on my end to debug, I'm happy to help (note that I wont be able to collect it till next week as I'm quite behind in my work this week...)
I suggest this issue/known bug of mqtt in node with MQTT v3.1.1 is documented because I'm guessing other people will scratch their head with this.