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

## 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:

```json
{
"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('')`.
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.