session Duplication

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

4.26.0

### Plugin version

10.7.0

### Node.js version

20.11.0

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

14.2.1

### Description

When the client creates a session:
**Services are multi-process, requests are concurrent and arrive at almost the same time, and multiple seesion ids may be created.**

There is a time difference between determining the need to create and the creation process.

They have a slight time difference after creation, but they can be seen as duplicates. saveUninitialized there will be this repetition whether true or false.

### Link to code that reproduces the bug

_No response_

### Expected Behavior

_No response_