Inlining schemas with json-schema-ref-parser is unnecessary, and may not work in case inlined schemas have $id

I think this might be a V7 bug as my schemas were all passing with V6.

If an external schema is referenced using `$ref` by more than one property, or within a property defined in the `definitions` section, and the `$refs` are resolved by putting the schemas inline, as with [json-schema-ref-parser](https://github.com/APIDevTools/json-schema-ref-parser), then compilation fails with the error

```
reference "https://schemas.example.com/samples/child" resolves to more than one schema
```

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
7.0.4

**Ajv options object**
{}

**JSON Schema**

### samples/parent.schema.json
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "https://schemas.example.com/samples/parent",
"title": "Schema that references another schema multiple times",
"type": "object",
"properties": {
"name": {
"type": "string"
},
"eldestChild": {
"$ref": "./child.schema.json"
},
"youngestChild": {
"$ref": "./child.schema.json"
}
}
}
```

### samples/child.schema.json
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "https://schemas.example.com/samples/child",
"title": "Schema referenced from a parent",
"description": "Example description",
"type": "object",
"properties": {
"name": {
"type": "string"
}
}
}
```

It also happens if the `$ref` only appears once in the file but it's inside an object in the definitions section:

### samples/parent2.schema.json
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "https://schemas.example.com/samples/parent2",
"title": "Schema that references another schema",
"description": "Example description",
"type": "object",
"properties": {
"name": {
"type": "string"
},
"children": {
"type": "array",
"items": {
"$ref": "#/definitions/child"
}
}
},
"definitions": {
"child": {
"type": "object",
"properties": {
"order": {
"type": "number"
},
"details": {
"$ref": "./child.schema.json"
}
}
}
}
}
```

This seems to be because `traverse()` finds it multiple times.

**Your code**

```javascript
const Ajv = require('ajv').default
const $RefParser = require('json-schema-ref-parser')

const ajv = new Ajv();

$RefParser.dereference("samples/parent.schema.json", function(err, schema) {
if (err) throw err
try {
ajv.compile(schema);
console.log('[ PASS ] Schema validation passed.')
}
catch (e) {
console.log(`[ FAIL ] Schema validation failed: "${e.message}"]`)
process.exit(1)
}
})
```

**Validation result, data AFTER validation, error messages**

```
[ FAIL ] Schema validation failed: "reference "https://schemas.example.com/samples/child" resolves to more than one schema"]
```

**What results did you expect?**
Schemas pass compilation. It's perfectly valid to reference an external schema multiple times or from inside local definitions, and it used to work fine.

**Are you going to resolve the issue?**
Maybe, but I'm not quite sure where to start. It seems like it's not necessary to throw an error if a schema is referenced more than once, since it didn't used to. But I assume that code was added for a reason, and I don't know what that reason is, or if anything bad would happen if it were removed.