"Item cannot come after itself" from ref with a duplicated property name

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): yes

#### Context

* *node version*: 14.20.1
* *module version with issue*: 17.7.0
* *last module version without issue*: not tested
* *environment* (e.g. node, browser, native): Node.js
* *used with* (e.g. hapi application, another framework, standalone, ...): Hapi
* *any other relevant information*:

#### What are you trying to achieve or the steps to reproduce?

I'm using a Knex ref with a nested key with a duplicate name.

For example:

```js
const schema = Joi.object({
id: Joi.number().required(),
command: Joi.object({
command: Joi.string().valid('run', 'jump').required(),
params: Joi.alternatives().conditional('command', {
switch: [
{
is: 'run',
then: Joi.object({ howFast: Joi.number().required() }),
},
{
is: 'jump',
then: Joi.object({ howHigh: Joi.number().required() }),
},
],
}),
})
});
```

A sample of a message that complies with this:

```js
{
id: 1,
command: {
command: 'run',
params: { howFast: 1 },
}
}
```

#### What was the result you got?

An error message is thrown within `@hapi/topo`:

> Item cannot come after itself: command

#### What result did you expect?

Usable schema

I'm working around this by renaming one of my property keys.