correctly remove duplicated secrets and ssh keys

[crazy-max]

• follow-up bake: support += operator to append with overrides #3031 (comment)

target "default" {
  secret = [
    "id=FOO,env=FOO",
    "id=FOO,env=BAR",
    "id=BAR,env=BAZ"
  ]
}

Currently:

$ docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 103B / 103B done
#1 DONE 0.0s
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "secret": [
        {
          "id": "FOO",
          "env": "FOO"
        },
        {
          "id": "FOO",
          "env": "BAR"
        },
        {
          "id": "BAR",
          "env": "BAZ"
        }
      ]
    }
  }
}

With this change:

$ docker buildx bake --print
#1 [internal] load local bake definitions
#1 reading docker-bake.hcl 103B / 103B done
#1 DONE 0.0s
{
  "group": {
    "default": {
      "targets": [
        "default"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "secret": [
        {
          "id": "FOO",
          "env": "BAR"
        },
        {
          "id": "BAR",
          "env": "BAZ"
        }
      ]
    }
  }
}

[jabidahscreationssystems]

All set to go.