In Swift, how can I make an attributed string only italicize the second instance of a word in a string?

You can write something like this:

                var atext: AttributedString = "The little duck marveled at the sight of the bigger ducks playing in the lake"
                if let range = atext.range(of: "duck") {
                    atext[range].font = .body.italic()
                }

For a simple example, building off of the answer above, to italicize the second instance of the substring "duck", you could do something like

if let firstRange = atext.range(of: "duck") {
    if let secondRange = atext[firstRange.upperBound...].range(of: "duck") {
        atext[secondRange].font = .body.italic()
    }
}

But as an important side note: in a real-world context, writing code like this does not lend well to localization if the source of your attributed string may be in a different language. If your attributed string is localized text you are displaying to a user, you could use markdown in your .strings file to indicate specific words that should be emphasized. You can see the "Markdown Support" section of this document for more details.