Double-Meanings
The Duchess of Malfi is rife with all kinds of doubles—from the mannequins of her family that Ferdinand makes to scare the Duchess to the two-facedness of Bosola to the fact that the Duchess and Ferdinand are actually twins.
It doesn't stop there, though: Webster's language is filled with double-meanings, and characters frequently speak to each other in a way that conveys both a surface meaning and a completely different underlying message.
Take, for example, the part of Act 3 Scene 2 where the Duchess, realizing she has to get Antonio out of the court now that Ferdinand's onto her, pretends to fire him in front of her courtiers.
The Duchess starts out at line 181, saying to Antonio, "I have got well by you," meaning, "you did a good job as steward in the past" and, secretly, "I like all the stuff that comes with being married to you, like our kids." They continue back and forth for the next 20 or so lines, all the way down to Antonio's exiting lines: "You may see, gentlemen, what 'tis to serve / A prince with body and soul" (3.2.205-6) meaning, to the courtiers, "Wow, this is what I get for being a good employee" and meaning, to the Duchess, "I—eeee—I will alwayyyys love you."
The entire conversation looks, from an outsider's point of view, like the Duchess is accusing Antonio of messing around with her finances and having done a bad job as her steward, while they're also having a private conversation about how much they love each other. Sneaky, right?
Blank Verse
Like most of his contemporaries, Webster wrote his plays largely in blank verse, which isn't actually blank at all: blank verse just refers to unrhymed iambic pentameter.
And what, pray tell, is iambic pentameter? Worry not, it's a lot less complicated than it sounds. Iambic pentameter is a metrical form wherein every line has ten syllables, and each unstressed syllable is followed by a stressed one. It comes out sounding like this: daDUM daDUM daDUM daDUM daDUM.
Adding some actual words, though, it looks like this:
miseRAble AGE where ONly THE reWARD
of DOing WELL is THE doING of IT. (1.1.31-32)
For Your Edification, a Note on Webster's Metrical Sneakiness
There are plenty of early modern authors who wrote in long, unbroken swathes of perfectly measured, beautifully written blank verse.
Webster ain't one of them.
For instance, if you're reading this play (instead of seeing/hearing it performed) you'll frequently come across a line that looks like it has way more than 10 syllables, and think, "hey, Webster, you totally cheated, that's not pentameter!"
But you have to keep in mind that the syllables of a word often get shmooshed together when spoken aloud. For example, "miserable" in the line above should properly have four syllables (mis-er-a-ble) but it gets spoken as having only three (mis-ra-ble), so the meter does in fact work out here.
That said, a lot of the time it doesn't work out, so you kind of have to chill out when trying to scan Webster's verse because it's so frequently irregular, and if you're too worried about the meter you'll end up devoting a lot time to a linguistic contortion act. You just might pull a muscle.
Additionally, Webster's metrical irregularity is often exactly what makes his work so powerful. Take, for instance, the moment where Ferdinand, standing over the Duchess's freshly executed body, says, "Cover her face. Mine eyes dazzle. She died young" (4.2.254). This line scans really, really badly as iambic pentameter, but wow does it pack a punch.
The departure from conventional blank verse—the starkness of the language, paired with metrical rule-breaking—result in this uncomfortably disjointed, utterly piercing collection of words that, together, constitute one of the most famous lines Webster ever wrote. In short, it stands out.
Prose
And then there's some prose, which is just people talking in the way that normal people normally do, instead to speaking in verse. There's not a ton of it in The Duchess of Malfi, but it does happen, usually in the form of Bosola talking smack about somebody. Like when he's harassing the Old Lady:
There was a lady in France that, having had the smallpox,
flayed the skin off her face to make it more level; and
whereas before she looked like a nutmeg grater, after she
resembled an abortive hedgehog. (2.1.26-29)