DNS Rewrites, duplicates items... maybe bug

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to report a bug and not [ask a question or ask for help](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

Linux, AMD64 (aka x86_64)

### Installation

GitHub releases or script from README

### Setup

On one machine

### AdGuard Home version

master

### Action


### DNS rewrites
Allows to easily configure custom DNS response for a specific domain name.

in this web ,, background code

```go
// handleRewriteAdd is the handler for the POST /control/rewrite/add HTTP API.
func (d *DNSFilter) handleRewriteAdd(w http.ResponseWriter, r *http.Request) {
```


Why not use map to remove duplicates dns rewrites items.
or
before append ,,check if exist...

why not

???

### Expected result

before append ,,check if exist...

### Actual result

so many duplicates item.

### Additional information and/or screenshots

_No response_