Key usages should be an unique set

[ChALkeR]

> const k = await crypto.subtle.exportKey('jwk', await crypto.subtle.generateKey({name: "ChaCha20-Poly1305" }, true, ['encrypt']))
> await crypto.subtle.importKey('jwk', k, "ChaCha20-Poly1305", true, ['encrypt', 'encrypt'])
CryptoKey {
 type: 'secret',
 extractable: true,
 algorithm: { name: 'ChaCha20-Poly1305' },
 usages: [ 'encrypt', 'encrypt' ] // duplicated
}
> await crypto.subtle.exportKey('jwk', _)
{
 key_ops: [ 'encrypt', 'encrypt' ], // duplicated
 ext: true,
 alg: 'C20P',
 kty: 'oct',
 k: '...'
}

> const k = await crypto.subtle.exportKey('jwk', await crypto.subtle.generateKey({name: "AES-GCM", length: 256 }, true, ['encrypt']))
undefined
> await crypto.subtle.importKey('jwk', k, {name: "AES-GCM", length: 256 }, true, ['encrypt', 'encrypt'])
CryptoKey {
 type: 'secret',
 extractable: true,
 algorithm: { name: 'AES-GCM', length: 256 },
 usages: [ 'encrypt', 'encrypt' ] // duplicated
}
> await crypto.subtle.exportKey('jwk', _)
{
 key_ops: [ 'encrypt', 'encrypt' ], // duplicated
 ext: true,
 alg: 'A256GCM',
 kty: 'oct',
 k: '...'
}

> k = await crypto.subtle.generateKey({ name: "RSA-OAEP", hash: "SHA-256", modulusLength: 2048, publicExponent: Uint8Array.of(1, 0, 1) }, true, [ 'encrypt', 'decrypt'])
> p = await crypto.subtle.exportKey('jwk', k.publicKey)
> await crypto.subtle.importKey('jwk', p, { name: "RSA-OAEP", hash: "SHA-256" }, true, ['encrypt', 'encrypt'])
CryptoKey {
 type: 'public',
 extractable: true,
 algorithm: {
 name: 'RSA-OAEP',
 modulusLength: 2048,
 publicExponent: [Uint8Array],
 hash: [Object]
 },
 usages: [ 'encrypt', 'encrypt' ] // duplicate
}
> await crypto.subtle.exportKey('jwk', _)
{
 key_ops: [ 'encrypt', 'encrypt' ], // duplicate
 ext: true,
 alg: 'RSA-OAEP-256',
 kty: 'RSA',
 n: '...',
 e: 'AQAB'
}

Chrome deduplicates:

Source:

chacha20_poly1305

https://github.com/nodejs/node/blob/HEAD/lib/internal/crypto/chacha20_poly1305.js#L143

aes

https://github.com/nodejs/node/blob/HEAD/lib/internal/crypto/aes.js#L288

rsa

https://github.com/nodejs/node/blob/HEAD/lib/internal/crypto/rsa.js#L284

[ChALkeR]

I'm actually unsure if this is covered by the spec at all

All of Chrome, Safari, Firefox deduplicate though