Ion-checkbox with ngmodel issue

hi , i have a little issue with ion-chekbox , when i add a ngModel on it and select one value , all values are selected

<ion-list *ngFor="#keyValue of skills | Firebase1">
  <ion-item>
      {{keyValue.key}}
  </ion-item>
   <ion-item *ngFor="#value of keyValue.value">
<ion-label>{{value.text}}</ion-label>
<ion-checkbox [(ngModel)]="skill">{{value.id}}</ion-checkbox>
  </ion-item>
</ion-list>

this is my html code