Rules duplicate when moz-placeholder prefix added

[discountscott]

Everytime the autoprefixer is run on the following bit of CSS, the moz-placeholder prefixes are added above (even if they already exist).
postcss ./**/*.css -u autoprefixer --no-map --base . --dir ./

They are multiplying everytime it runs...

#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
    font-size: 2rem;
}

After a few runs in GitHub actions with the following browserslist > 0.5%, last 6 versions, Firefox ESR, not dead

you end up with the following:

#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
    font-size: 2rem;
}
#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
    font-size: 2rem;
}
#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
    font-size: 2rem;
}
#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
    font-size: 2rem;
}
#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
    font-size: 2rem;
}
#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
    font-size: 2rem;
}

[ai]

I can’t reproduce it on https://autoprefixer.github.io/

Maybe you are using some old Autoprefixer or have any other problem in environment.

Try to reproduce issue in s simplest new project.

[discountscott]

I can reproduce it there. Looks like it gets duplicated when it gets formatted to match our style (each on own line). If you feed that back in again, it'll add it again.

So if you enter

#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
	font-size: 2rem;
}

You'll get the output of

#cardNumber::-moz-placeholder, #cardHolder::-moz-placeholder, #expDate::-moz-placeholder, #cvc::-moz-placeholder {
	font-size: 2rem;
}
#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
	font-size: 2rem;
}

We format that with prettier to

#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
	font-size: 2rem;
}
#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
	font-size: 2rem;
}

Put that back in the autoprefixer and it'll generate

#cardNumber::-moz-placeholder,
#cardHolder::-moz-placeholder,
#expDate::-moz-placeholder,
#cvc::-moz-placeholder {
	font-size: 2rem;
}
#cardNumber::-moz-placeholder, #cardHolder::-moz-placeholder, #expDate::-moz-placeholder, #cvc::-moz-placeholder {
	font-size: 2rem;
}
#cardNumber::placeholder,
#cardHolder::placeholder,
#expDate::placeholder,
#cvc::placeholder {
	font-size: 2rem;
}

[ai]

Yes, Autoprefixer duplication prevention works only if you pass exact output.

Unfortunately, duplication prevention is not a priority for me. You will need to debug the code and send PR. Here is a block where you need to work https://github.com/postcss/autoprefixer/blob/main/lib/selector.js#L89-L119

Ask any questions if you will have them during the work.