Fix duplicate alternative backend merging

[wayt]

What this PR does / why we need it:
When there are multiple locations per server, where each location is configured to point to the same server/backend, ingress-nginx will merge alternative backend into the main one repeatedly.

Ex:

  {
    "name": "web-app-80",
    "service": {
      "metadata": {
        "creationTimestamp": null
      },

...

    "noServer": false,
    "trafficShapingPolicy": {
      "weight": 0,
      "header": "",
      "cookie": ""
    },
    "alternativeBackends": [
      "web-canary-app-80",
      "web-canary-app-80",
      "web-canary-app-80"
    ]
  },

Here, web-canary-app-80 gets merged 3 times while having 3 ingresses with different locations. All pointing to the same server web-app.

This causes no real issue for now but is definitely incorrect.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Test in controller_test.go TestMergeAlternativeBackends for this are irrelevant because ingress.Backend.Equal does not check for a duplicate value https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/types_equals.go#L182

[wayt]

/assign @ElvinEfendi

[ElvinEfendi]

/approve

I'll let @aledbf lgtm so that he can also take another look.

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi, wayt

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