### Elasticsearch Version

8.3.2

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Ubuntu 20.04.4 LTS

### Problem Description

When we have two legacy index templates and one of them defines the field name with dots (.), we may get mapper_parsing_exception.

### Steps to Reproduce

1. create two legacy index templates
```
PUT /_template/template1
{
  "order": 100,
  "index_patterns": [
    "haha*"
  ],
  "settings": {},
  "mappings": {
    "properties": {
      "kwm": {
        "properties": {
          "source": {
            "properties": {
              "geo": {
                "properties": {
                  "location": {
                    "type": "geo_point"
                  }
                }
              }
            }
          }
        }
      },
      "source": {
        "properties": {
          "geo": {
            "properties": {
              "location": {
                "type": "geo_point"
              }
            }
          }
        }
      }
    }
  }
}

PUT /_template/template2
{
  "order": 10,
  "index_patterns": [
    "haha*"
  ],
  "settings": {},
  "mappings": {
    "properties": {
      "kwm.source.geo": {
        "properties": {
          "location": {
            "type": "geo_point"
          }
        }
      }
    }
  }
}
```
3. try to create an index
```
POST haha/_doc
{
}
```
it produces `mapper_parsing_exception` in `8.3.2` as below
```
{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "Field [source.geo.location] is defined more than once"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "Field [source.geo.location] is defined more than once"
  },
  "status": 400
}
```
but no error in `8.2.3`

4. if we change the second index template `template2` as below:
```
PUT /_template/template2
{
  "order": 10,
  "index_patterns": [
    "haha*"
  ],
  "settings": {},
  "mappings": {
    "properties": {
      "kwm": {
        "properties": {
          "source": {
            "properties": {
              "geo": {
                "properties": {
                  "location": {
                    "type": "geo_point"
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}
```
we can create the `haha` index without errors in both `8.3.2` and `8.2.3`

I think we should not get `mapper_parsing_exception` in both `8.3.2` and `8.2.3` in the step 3 because `kwm.source.geo.location` field definition of two index templates are identical and `source.geo.location` is defined only in the `template1` index template.
I think this may be related with https://github.com/elastic/elasticsearch/pull/86166

### Logs (if relevant)

_No response_
server/src/test/java/org/elasticsearch/index/mapper/MapperServiceTests.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.index.mapper;
import org.elasticsearch.Version;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.common.bytes.BytesReference;
import org.elasticsearch.common.compress.CompressedXContent;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.index.mapper.MapperService.MergeReason;
import org.elasticsearch.indices.IndicesModule;
import org.elasticsearch.test.VersionUtils;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xcontent.XContentFactory;
import java.io.IOException;
import java.util.List;
import java.util.stream.StreamSupport;
import static org.hamcrest.CoreMatchers.containsString;
import static org.hamcrest.Matchers.containsInAnyOrder;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.notNullValue;
import static org.hamcrest.Matchers.nullValue;
public class MapperServiceTests extends MapperServiceTestCase {