Fixes by using track by.

src/Squidex/app/features/schemas/pages/schema/field.component.html

@@ -106,7 +106,7 @@
             <span class="nested-field-line-v"></span>
 
             <div [sqxSortModel]="nested" (sqxSorted)="sortFields($event)" handleClass="nested">
-                <div class="nested-field" *ngFor="let nested of nested">
+                <div class="nested-field" *ngFor="let nested of nested; trackBy: trackByField">
                     <span class="nested-field-line-h"></span>
     
                     <sqx-field [field]="nested" [schema]="schema" [parent]="field" [patterns]="patterns" handleClass="nested"></sqx-field>

src/Squidex/app/features/schemas/pages/schema/field.component.ts

@@ -111,6 +111,10 @@ export class FieldComponent implements OnInit {
         this.schemasState.lockField(this.schema, this.field).onErrorResumeNext().subscribe();
     }
 
+    public trackByField(index: number, field: NestedFieldDto) {
+        return field.fieldId;
+    }
+
     public save() {
         const value = this.editForm.submit();
 

src/Squidex/app/features/schemas/pages/schema/schema-page.component.html

@@ -61,7 +61,7 @@
 
         <ng-container *ngIf="patternsState.patterns | async; let patterns">
             <div class="schemas" [sqxSortModel]="schema.fields" (sqxSorted)="sortFields($event)" handleClass="root">
-                <sqx-field *ngFor="let field of schema.fields" [field]="field" [schema]="schema" [patterns]="patterns" handleClass="root"></sqx-field>
+                <sqx-field *ngFor="let field of schema.fields; trackBy: trackByField" [field]="field" [schema]="schema" [patterns]="patterns" handleClass="root"></sqx-field>
             </div>
     
             <button class="btn btn-success field-button" (click)="addFieldDialog.show()">

src/Squidex/app/features/schemas/pages/schema/schema-page.component.ts

@@ -88,6 +88,10 @@ export class SchemaPageComponent implements OnDestroy, OnInit {
         this.schemasState.sortFields(this.schema, fields).subscribe();
     }
 
+    public trackByField(index: number, field: FieldDto) {
+        return field.fieldId;
+    }
+
     public deleteSchema() {
         this.schemasState.delete(this.schema)
             .subscribe(() => {