[sdk/local-evaluation] Duplicate identity override entries silently drop feature overrides

[xolott-ark]

Problem

When an environment document returned by the Flagsmith API contains more than one entry in identity_overrides sharing the same identifier, the SDK keeps only the last entry for that identifier during local evaluation. Any identity_features defined on the earlier entries are discarded, so the corresponding flags fall back to the environment default instead of the configured override.

For example, given an environment payload like:

{
  "identity_overrides": [
    {
      "identifier": "multi-override-id",
      "identity_features": [
        {
          "feature": { "name": "some_feature" },
          "feature_state_value": "override-from-first-entry",
          "enabled": false
        }
      ]
    },
    {
      "identifier": "multi-override-id",
      "identity_features": [
        {
          "feature": { "name": "mv_feature" },
          "feature_state_value": "override-from-second-entry",
          "enabled": true
        }
      ]
    }
  ]
}

Calling getIdentityFlags('multi-override-id') with local evaluation enabled returns the override for mv_feature only. some_feature resolves to its environment default, as if the first entry never existed.

Impact

Any identifier whose overrides arrive split across multiple identity_overrides entries gets the wrong flag values during server-side local evaluation. The breakage is silent (no warning, no error) and deterministic for a given payload, so affected identifiers stay broken on every request until the environment payload changes shape.

Reproduction

1. Use an environment whose identity_overrides array contains two entries with the same identifier, each carrying a different feature in identity_features (e.g. entry A overrides some_feature, entry B overrides mv_feature).
2. Initialise the SDK with enableLocalEvaluation: true and call getIdentityFlags('<that-identifier>').
3. Observed: only the features from the last entry are applied. some_feature resolves to its environment default instead of "override-from-first-entry".
4. Expected: both some_feature and mv_feature resolve to their respective override values and enabled states.

[xolott-ark]

I created a PR with the fix #268