HTTP Interceptor send same request twice

I'm using an HTTP interceptor in order to add auth token to requests but when the http client fires the request, this is intercepted and sent twice

This is my HttpClient call

  searchProd(query: string, limit?: number): Observable<Product[]> {
    let _limit = limit || 5;
    return this.http.get<Product[]>(`${API_CONST.HOST}/${API_CONST.PRODUCT}?search=${query}&limit=${_limit}`);
  }

This is my app.module.ts

import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http'; 
import { TokenInterceptor } from './auth/token.interceptor';
....

@NgModule({
  declarations: [
    ...
  ],
  imports: [
    ...
  ],
  providers: [
    ApiService,
    AuthGuardService,
    SettingsService,
    {
      provide : HTTP_INTERCEPTORS,
      useClass : TokenInterceptor,
      multi : true
    }
  ],
  entryComponents: [ ... ],
  bootstrap: [ ... ]
})

This is my token.interceptor.ts

import { Injectable } from '@angular/core';
import {
  HttpRequest,
  HttpHandler,
  HttpEvent,
  HttpInterceptor,
  HttpResponse,
  HttpErrorResponse
} from '@angular/common/http';
import { AngularFireAuth } from '@angular/fire/auth';
import { Observable } from 'rxjs';
import { AuthGuardService } from './auth-guard.service';
import { API_CONST } from '../services/api/api.service';

@Injectable()
export class TokenInterceptor implements HttpInterceptor {
  private no_auth_endpoints = [
    `${API_CONST.HOST}/${API_CONST.PRODUCT}`
  ]
  private token = null;
  constructor(public af: AngularFireAuth, private authGuard: AuthGuardService) {
    this.token = authGuard.getToken();
  }
  intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    const headersConfig = {
      'Authorization': `Bearer ${this.token}`
    };

    let isAuthEnpoint = true;
    this.no_auth_endpoints.forEach(endpoint => {
      if(request.url.includes(endpoint))
        isAuthEnpoint = false;
    })

    if (!request.headers.has('Authorization') && isAuthEnpoint) {
      const modified = request.clone({
        setHeaders : headersConfig
      });

      return next.handle(modified); //this line is executed twice!
    }
    else {
      return next.handle(request); //this line is executed twice!
    }

  }
}

Throught the chrome dev tools i see the same request sent twice in the network tab. During debugging I saw the http request send by searchProd once but when it's intecepted the next.handle() is executed twice. How to fix that in order to send only one request?

EDIT: This is what is shown in network tab

First request

Second request

EDIT2: This is the code where I call the searchProd(string) function.

component.html

<mat-form-field class="bottom-search-field">
    <input [formControl]="autoCompleteControl" type="text" placeholder="Aggiungi un prodotto"
      matInput [matAutocomplete]="auto">
    <mat-autocomplete autoActiveFirstOption #auto="matAutocomplete" (optionSelected)="onSelectionChanged($event)">
      <mat-option *ngFor="let item of searchResults | async; let index = index" [value]="item.description | titlecase">
        {{ item.description | titlecase}}
      </mat-option>
    </mat-autocomplete>
  </mat-form-field>

component.ts

public autoCompleteControl = new FormControl();
...
ngOnInit(): void {
    this.searchResults = this.autoCompleteControl.valueChanges.pipe(
      startWith(''),
      switchMap(value => {
        if (value.length > 3) {
          let prodSearched = this.apiService.searchProd(value);
          prodSearched.pipe(share()); // ADDED PIPE SHARE
          this.saveProdSearched(prodSearched);
          return prodSearched;
        } else {
          return of(null);
        }
      })
    );
  }
  //This function save the last result inside of an array of Product
  private saveProdSearched(prodSearched: Observable<Product[]>) {
    prodSearched.subscribe(items => {
      this.lastSearch = items
    })
  }