Riff-raff.—The refuse of anything, "Il ne lui lairra rif ny raf." Cotgrave in Rif, where rif is said to mean nothing.
Rolly-pooly.—"A sort of game" (Johnson). It is now used as the name of a pudding rolled with sweetmeat.
Rowdy-dowdy, and Rub-a-dub.—Words formed in imitation of the beat of a drum.
Shilly-shally.—Used by Congreve, and formerly written "shill I, shall I."
Slip-slop.—"Bad liquor. A low word, formed by reduplication of slop" (Johnson). Now generally applied to errors in pronunciation, arising from ignorance and carelessness, like those of Mrs. Malaprop in The Rivals.
Tip-top.—Formed from top, like slip-slop from slop.
Tirra-lirra.—Used by Shakspeare:
"The lark that tirra lirra chants."—Winter's Tale, Act IV. Sc. 2.
From the French, see Nares in v.
The preceding collection is intended merely to illustrate the principle upon which this class of words are formed, and does not aim at completeness. Some of your correspondents will doubtless, if they are disposed, be able to supply other examples of the same mode of formation.