fix(tasks): don't allow adding the same assignee multiple times

See https://community.vikunja.io/t/task-can-be-assigned-twice-or-more-to-the-same-user/883

src/components/tasks/partials/editAssignees.vue

@@ -28,7 +28,7 @@
 </template>
 
 <script setup lang="ts">
-import {ref, shallowReactive, watch, type PropType} from 'vue'
+import {ref, shallowReactive, watch, nextTick, type PropType} from 'vue'
 import {useI18n} from 'vue-i18n'
 
 import User from '@/components/misc/user.vue'
@@ -67,6 +67,7 @@ const {t} = useI18n({useScope: 'global'})
 const listUserService = shallowReactive(new ListUserService())
 const foundUsers = ref([])
 const assignees = ref<IUser[]>([])
+let isAdding = false
 
 watch(
 	() => props.modelValue,
@@ -80,9 +81,19 @@ watch(
 )
 
 async function addAssignee(user: IUser) {
+	if (isAdding) {
+		return
+	}
+
+	try {
+		nextTick(() => isAdding = true)
+
 		await taskStore.addAssignee({user: user, taskId: props.taskId})
 		emit('update:modelValue', assignees.value)
 		success({message: t('task.assignee.assignSuccess')})
+	} finally {
+		nextTick(() => isAdding = false)
+	}
 }
 
 async function removeAssignee(user: IUser) {
@@ -119,6 +130,7 @@ function clearAllFoundUsers() {
 }
 
 const multiselect = ref()
+
 function focus() {
 	multiselect.value.focus()
 }

src/stores/tasks.ts

@@ -171,6 +171,9 @@ export const useTaskStore = defineStore('task', {
 			user: IUser,
 			taskId: ITask['id']
 		}) {
+			const cancel = setModuleLoading(this)
+			
+			try {
 				const kanbanStore = useKanbanStore()
 				const taskAssigneeService = new TaskAssigneeService()
 				const r = await taskAssigneeService.create(new TaskAssigneeModel({
@@ -196,7 +199,11 @@ export const useTaskStore = defineStore('task', {
 						],
 					},
 				})
+
 				return r
+			} finally {
+				cancel()
+			}
 		},
 
 		async removeAssignee({