Duplicate coordinates throwing `No cluster with the specified ID`

Nobody has claimed this yet.

We are using Supercluster wrapped in https://github.com/novalabio/react-native-maps-super-cluster

On loading our map, we've experienced the `No cluster with the specified ID` error when clicking on a cluster. This was happening within the following call to `getLeaves()`, where `clusteringEngine` is Supercluster.
```
clusteringEngine
.getLeaves(clusterId, 100)
.map(c => c.properties.item);
```

It turns out the error happened when a cluster had two points that shared exactly the same coordinates, leading to the below stack trace, where I log out from the `getChildren()` function. As you can see, it thinks the child of the cluster is also a cluster at the same coordinates, and the child of that cluster is also apparently a cluster at the same coordinates, but that 'cluster' doesn't have any children IDs so the error is thrown.

I'm wondering:
1) whether Supercluster is already meant to handle multiple points at the same coordinates, in which case is this a bug?; and
2) whether in the event that `getChildren` _cannot_ find the children for a cluster, an empty array should be returned rather than an error thrown. Or perhaps `_appendLeaves` should set `children` to an empty array if the error gets thrown, like this:
```
_appendLeaves: function (result, clusterId, limit, offset, skipped) {
try {
var children = this.getChildren(clusterId);
} catch(error){
if (error.message.includes('No cluster with the specified id')) {
children = []
} else {
throw error
}
}
...
```

**Stack trace:**
```
2018-10-19 21:27:40.887 [info][tid:com.facebook.react.JavaScript] ------------------------ get children
2018-10-19 21:27:40.887170+0100 Refill[68457:2682417] ------------------------ get children
2018-10-19 21:27:40.887 [info][tid:com.facebook.react.JavaScript] 'clusterId', 368017
2018-10-19 21:27:40.887494+0100 Refill[68457:2682417] 'clusterId', 368017
2018-10-19 21:27:40.888 [info][tid:com.facebook.react.JavaScript] '------------------------ ids', [ 11500 ]
2018-10-19 21:27:40.887822+0100 Refill[68457:2682417] '------------------------ ids', [ 11500 ]
2018-10-19 21:27:40.888 [info][tid:com.facebook.react.JavaScript] 'c.parentId', 368017
2018-10-19 21:27:40.888069+0100 Refill[68457:2682417] 'c.parentId', 368017
2018-10-19 21:27:40.888 [info][tid:com.facebook.react.JavaScript] push into children array
2018-10-19 21:27:40.888282+0100 Refill[68457:2682417] push into children array
2018-10-19 21:27:40.889 [info][tid:com.facebook.react.JavaScript] 'children', [ { type: 'Feature',
properties:
{ cluster: true,
cluster_id: 385970,
point_count: 2,
point_count_abbreviated: 2 },
geometry:
{ type: 'Point',
coordinates: [ -5.538370000000008, 50.11913999999999 ] } } ]
2018-10-19 21:27:40.888730+0100 Refill[68457:2682417] 'children', [ { type: 'Feature',
properties:
{ cluster: true,
cluster_id: 385970,
point_count: 2,
point_count_abbreviated: 2 },
geometry:
{ type: 'Point',
coordinates: [ -5.538370000000008, 50.11913999999999 ] } } ]
2018-10-19 21:27:40.889 [info][tid:com.facebook.react.JavaScript] ------------------------ get children
2018-10-19 21:27:40.888954+0100 Refill[68457:2682417] ------------------------ get children
2018-10-19 21:27:40.889 [info][tid:com.facebook.react.JavaScript] 'clusterId', 385970
2018-10-19 21:27:40.889217+0100 Refill[68457:2682417] 'clusterId', 385970
2018-10-19 21:27:40.889 [info][tid:com.facebook.react.JavaScript] '------------------------ ids', [ 12061 ]
2018-10-19 21:27:40.889467+0100 Refill[68457:2682417] '------------------------ ids', [ 12061 ]
2018-10-19 21:27:40.890 [info][tid:com.facebook.react.JavaScript] 'c.parentId', 385970
2018-10-19 21:27:40.889677+0100 Refill[68457:2682417] 'c.parentId', 385970
2018-10-19 21:27:40.890 [info][tid:com.facebook.react.JavaScript] push into children array
2018-10-19 21:27:40.889931+0100 Refill[68457:2682417] push into children array
2018-10-19 21:27:40.890 [info][tid:com.facebook.react.JavaScript] 'children', [ { type: 'Feature',
properties:
{ cluster: true,
cluster_id: 388691,
point_count: 2,
point_count_abbreviated: 2 },
geometry:
{ type: 'Point',
coordinates: [ -5.538370000000008, 50.11913999999999 ] } } ]
2018-10-19 21:27:40.890418+0100 Refill[68457:2682417] 'children', [ { type: 'Feature',
properties:
{ cluster: true,
cluster_id: 388691,
point_count: 2,
point_count_abbreviated: 2 },
geometry:
{ type: 'Point',
coordinates: [ -5.538370000000008, 50.11913999999999 ] } } ]
2018-10-19 21:27:40.891 [info][tid:com.facebook.react.JavaScript] ------------------------ get children
2018-10-19 21:27:40.890627+0100 Refill[68457:2682417] ------------------------ get children
2018-10-19 21:27:40.891 [info][tid:com.facebook.react.JavaScript] 'clusterId', 388691
2018-10-19 21:27:40.890848+0100 Refill[68457:2682417] 'clusterId', 388691
2018-10-19 21:27:40.891 [info][tid:com.facebook.react.JavaScript] '------------------------ ids', []
2018-10-19 21:27:40.891095+0100 Refill[68457:2682417] '------------------------ ids', []
2018-10-19 21:27:40.891 [info][tid:com.facebook.react.JavaScript] 'children', []
2018-10-19 21:27:40.891329+0100 Refill[68457:2682417] 'children', []
2018-10-19 21:27:42.898 [warn][tid:com.facebook.react.JavaScript] Possible Unhandled Promise Rejection (id: 0):
Error: No cluster with the specified id.
```

Start with getLeaves(), getChildren(), and _appendLeaves(), using the reported duplicate-coordinate scenario and stack trace to trace how child cluster IDs are generated and traversed. Done means the scenario is handled consistently without producing an invalid child ID or the “No cluster with the specified id” error; the issue does not name a test file.

Written by the indexing model from the issue text.