Today, I thought I lost the ability to merge tasks as duplicates. When writing a question in English Forum > Ask the Community the forum helpfully suggested this thread: Merge Duplicate Tasks - where did it go?
TLDR: it doesn’t work on recurring tasks.
If the amount of views on that thread (2.7K) is any reliable indication, there are more people that had this issue.
I think this can - and should - be made a lot more obvious from the interface. For example by greying it out and providing a tooltip that explains why the option is not available, like with rule triggers:
