Dynamic select in Materialize / Angular

I’m trying to fill a select in materialize with info from my db but I get a warning and the select is not filled.

I’m not able to upload images so I write the warning:

[DOM] Found 5 elements with non-unique id #op1:

(This appears according the number of registers)

<input _ngcontent-xir-c52 id=​"op1" formcontrolname=​"field1" type=​"radio" class=​"with-gap ng-untouched ng-pristine ng-valid" ng-reflect-form-control-name=​"field1" ng-reflect-name=​"field1" ng-reflect-value=​"5f66534d15ee4c34b4239710">​

Here is the select code:

<div class="input-field col s12">
   <option value="" disabled selected>Choose your option</option>
       <ng-container *ngFor="let f of fields">
          <option [value]="f._id">{{f.name}}</option>


Source: Angular Questions