Remova duplicatas do Array Angular
const result = Array.from(this.item.reduce((m, t) => m.set(t.name, t), new Map()).values());
Xerothermic Xenomorph
const result = Array.from(this.item.reduce((m, t) => m.set(t.name, t), new Map()).values());<option *ngFor="let i of taxlist | unique" [value]="i.tax_name {{i.tax_name}} ( {{i.tax_percentage}}% )</option>