Problem with do_quote_ident()

I see in quote.c::do_quote_ident():
*cp2++ = '"';while (len-- > 0){    if (*cp1 == '"')        *cp2++ = '"';    if (*cp1 == '\\')        *cp2++ = '\\';
*cp2++= *cp1++;}*cp2++ = '"';
 

I am confused by the backslash handling. In my tests, a backslash in a
double-quoted string does not require two backslashes:test=> create user "a\d";CREATE USERtest=> select usename,
length(usename)from pg_user; usename  | length ----------+-------- a\d      |      3
 

This is because a double-quote in a double-quoted string is entered as
"", not \".

Is it adding another backslash because it assumes the result will appear
in another quoted string?

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