[Viz] legend duplicates percentile options when chart has both left & right Y axes

[dmlemeshko]

Kibana version: latest master / 7.14.0 on cloud

Elasticsearch version: 8.0-SNAPSHOT / 7.14.0 on cloud

Server OS version: MacOS 11.4

Browser version: Chrome 93.0.4577.63

Browser OS version: MacOS 11.4

Original install method (e.g. download page, yum, from source, etc.): from source / cloud instance for 7.14.0

Describe the bug: legend duplicates percentile options when chart has left+right Y axes

Steps to reproduce:

1. Create new Vertical Bar Chart
2. Add 2 metrics (e.g. Avg & Mean) with different fields on Y axis, add Date Histogram on X axis
3. Add Right axis for 2nd metric on Metrics & Axes tab
4. Add 3rd metric as percentile (e.g. with 3 options 75, 95, 99) with one of 2 fields you picked previously
5. Update chart

Expected behavior: Legend should display only unique options based on selected metrics

Screenshots (if relevant):

Viz_bug.mov

Inspect panel does not show duplicates

If you drag percentile agg to the top / middle, duplication is fixed but last option (99) is gone from legend

Viz.Legend.after.agg.dragging.mov

Errors in browser console (if relevant): no errors

Provide logs and/or server output (if relevant):

Any additional context:

Request from Inspect panel:

{
  "aggs": {
    "2": {
      "date_histogram": {
        "field": "timestamp",
        "calendar_interval": "1d",
        "time_zone": "Europe/Berlin",
        "min_doc_count": 1
      },
      "aggs": {
        "1": {
          "max": {
            "field": "requestTime"
          }
        },
        "3": {
          "percentiles": {
            "field": "maxUsers",
            "percents": [
              50
            ]
          }
        },
        "5": {
          "max": {
            "field": "requestTime"
          }
        },
        "6": {
          "percentiles": {
            "field": "requestTime",
            "percents": [
              75,
              95,
              99
            ],
            "keyed": false
          }
        }
      }
    }
  },
  "size": 0,
  "fields": [
    {
      "field": "requestSendStartTime",
      "format": "date_time"
    },
    {
      "field": "responseReceiveEndTime",
      "format": "date_time"
    },
    {
      "field": "timestamp",
      "format": "date_time"
    }
  ],
  "script_fields": {},
  "stored_fields": [
    "*"
  ],
  "runtime_mappings": {},
  "_source": {
    "excludes": []
  },
  "query": {
    "bool": {
      "must": [],
      "filter": [
        {
          "match_phrase": {
            "scenario.keyword": "org.kibanaLoadTest.simulation.branch.DemoJourney"
          }
        },
        {
          "range": {
            "timestamp": {
              "gte": "2021-08-29T01:08:56.327Z",
              "lte": "2021-09-14T20:47:53.609Z",
              "format": "strict_date_optional_time"
            }
          }
        }
      ],
      "should": [],
      "must_not": []
    }
  }
}

[elasticmachine]

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