(insert ?\n) spuriously calls before-change-functions twice. Help!

[Alan Mackenzie]

Hi, Emacs.

The following is a partial diagnosis of a bug reported by an AWK user.
(AWK Mode throws an error when require-final-newline is t).

In emacs -Q, in an AWK buffer, do M-: (insert ?\n).  The following
happens:
1. before-change-functions is invoked with arguments (beg end) (16 16).
2. The NL is inserted at position 16.
3. before-change-functions is invoked with arguments (16 17).
This all happens before after-change-functions is invoked.

However, M-: (insert "\n") just does the Right Thing.

This second invocation of before-change-functions is surely a bug.  I
can make this happen in a Fundamental Mode buffer in my normal Emacs
session, but not in Emacs -Q.

What is causing this spurious invocation?  Does anybody have any idea?
I've perused the C source for `insert', without finding anything.  Could
it be anything to do with text-properties?

Thanks for the help!

-- 
Alan Mackenzie (Nuremberg, Germany).