Odd one. I remember having this bug a while back, but I've been getting it now, so might as well report it.
Every time a task group of mine completes it's orders, for some inane reason, it repeats the message an absolute ridiculous amount.
This seems to be affecting TGs with little rhyme or reason, and hasn't affected ships that I've put to actually loop their orders.
To clarify, this is just a specific 1 ship task group in this screenshot, which shares no names with any other task groups, and of this kind I only run a few anyway. It is very strange, and might be a bit frustrating having to filter out "orders complete" as I will likewise then miss pace with a lot of my fleet operations.