Key usages should be an unique set

> 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'
}