Because 2 Unicode characters by a single keystroke just happens when you press a dead key twice.
It was 9 years ago when I ran into this issue reported by a customer.
Apparently a particular IME generated a surrogate pair, 2 Unicode characters for a single keystroke.
In this case you don't get a WM_KEYDOWN for the second charcter but you do get another WM_CHAR.
We need to find out which IME does this and which keystroke to test this.