Green’s Dictionary of Slang

ditto n.

also dittoes
[SE ditto, the same. The style is common today, but less so when the sl. was coined]

a suit of clothes (jacket, waistcoat, trousers) all the same colour.

[UK]Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: A suit of ditto; coat, waistcoat, and breeches, all of one colour.
[UK]Lex. Balatronicum.
[UK]T. Taylor Ticket-Of-Leave Man Act II: In a suit of grey dittoes, a-stepping out with another chap.
[UK] ‘’Arry on the River’ in Punch 9 Aug. 57/1: You’d a thought me a Turk in check dittos, and looked on your ’Arry with pride.
[UK]H. Smart Social Sinners I 201: A slight, dark man, of middle height, clad in an ordinary suit of dittoes, entered the room.
[Aus]‘Rolf Boldrewood’ Colonial Reformer I 11: The labourers wore the same shoddy suits [...] the shabby-genteel people the same threadbare ditto.
[UK]G.A. Sala London Up to Date 62: I should respectfully advise him [...] not to be in the habit of perambulating Pall Mall in a suit of dittoes and a pot hat.