So "\u1234" creates a string with one character '\u1234' in it. "\uu1234" however creates the string "\u1234" with 6 characters in it. The "\uu" sequence gets turned into "\u". This is intentional, as the code in the lexer says

/* A double-u quoted escape. Convert the "\u" or "\U" to "", * thereby shaving off a "u" or "U" from the escape * sequence. */

Where does this weird syntax come from? Why is it desired?