Multiple categories of a post metadata will duplicate a repeated category tag

Just like the following image shown:

![image](https://user-images.githubusercontent.com/110872846/198170824-84639238-3743-4388-989e-285a30b3409f.png)


Post A:
```
categories: [ networking, programming ]
```
Post B:
```
categories: [ programming ]
```

they should be merged !