Diga lo que diga la contraportada, quiero leer el prólogo antes de comprar el libro.

Whatever the back cover says, I want to read the prologue before buying the book.

Breakdown of Diga lo que diga la contraportada, quiero leer el prólogo antes de comprar el libro.

• yo: I
• querer: to want
• el libro: the book
• leer: to read

Questions & Answers about Diga lo que diga la contraportada, quiero leer el prólogo antes de comprar el libro.

Why is diga used twice in Diga lo que diga?

This is a very common Spanish pattern:

diga lo que diga = whatever ... says / no matter what ... says

The verb is repeated to create a concessive meaning: it shows that the result in the main clause does not change.

So here:

• Diga lo que diga la contraportada
  = Whatever the back cover says = No matter what the back cover says

This structure often uses the present subjunctive:

• haga lo que haga = whatever he/she does
• vayas donde vayas = wherever you go
• sea como sea = however it may be

So the repetition is not accidental; it is part of a set pattern.

Is diga here an imperative, or is it the subjunctive?

Here it is the present subjunctive, not an imperative.

Why?

Because the sentence is not giving a command. It is using the pattern:

• diga lo que diga = whatever it says

The imperative diga would mean say! when speaking formally to usted, but that meaning does not fit here.

A good clue is the full structure:

• diga lo que diga + noun/pronoun

That strongly points to the concessive subjunctive pattern.