Issue #12834 has been updated by Nikola Nenkov.


My point was that whether or not a module will get prepended can have an actual impact, yet it got changed between versions without an explicit notice. Given that the behaviour is documented as opposed to left undefined, I thought that this is a regression.

----------------------------------------
Bug #12834: `prepend` getting prepended even if it already exists in the ancestors chain
https://bugs.ruby-lang.org/issues/12834#change-60882

* Author: Nikola Nenkov
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 2.3.1
* Backport: 2.3: UNKNOWN
----------------------------------------
```
module M; end
class A; prepend M; end
class B < A; prepend M; end

B.ancestors # => [M, B, M, A, Object, Kernel, BasicObject]
```

Even though I find this behaviour to be more intuitive, it is inconsistent with `Module#include` and is potentially breaking.
I didn't see a mention on the [[release notes]](https://www.ruby-lang.org/en/news/2015/12/25/ruby-2-3-0-released/) and the [[documentation]](https://ruby-doc.org/core-2.3.1/Module.html#method-i-prepend_features) is now outdated. 



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