Remove duplicate users and filter out nulls

Two temporary FE workarounds are added for these issues:
https://gitlab.com/gitlab-org/gitlab/-/issues/329750
https://gitlab.com/gitlab-org/gitlab/-/issues/327822

app/assets/javascripts/vue_shared/components/user_select/user_select.vue

@@ -88,6 +88,7 @@ export default {
       },
     },
     searchUsers: {
+      errorPolicy: 'all',
       query: searchUsers,
       variables() {
         return {
@@ -97,10 +98,32 @@ export default {
         };
       },
       update(data) {
-        return data.workspace?.users?.nodes.map(({ user }) => user) || [];
+        if (!data.workspace?.users?.nodes) return [];
+
+        // TODO this de-duplication is temporary (BE fix required)
+        // Also remove the error policy and related test (mockdata)
+        // https://gitlab.com/gitlab-org/gitlab/-/issues/327822
+        const users = data.workspace?.users?.nodes.filter((x) => x).map(({ user }) => user);
+        return users.reduce((acc, cur) => {
+          return acc.find((u) => u.id === cur.id) ? acc : [...acc, cur];
+        }, []);
       },
       debounce: ASSIGNEES_DEBOUNCE_DELAY,
-      error() {
+      error({ graphQLErrors }) {
+        // TODO This error suppression is temporary (BE fix required)
+        // https://gitlab.com/gitlab-org/gitlab/-/issues/329750
+        if (
+          graphQLErrors.length === 1 &&
+          graphQLErrors[0]?.message === 'Cannot return null for non-nullable field GroupMember.user'
+        ) {
+          // eslint-disable-next-line no-console
+          console.error(
+            "Suppressing the error 'Cannot return null for non-nullable field GroupMember.user'. Please see https://gitlab.com/gitlab-org/gitlab/-/issues/329750",
+          );
+          this.isSearching = false;
+          return;
+        }
+
         this.$emit('error');
         this.isSearching = false;
       },

spec/frontend/sidebar/mock_data.js

@@ -404,6 +404,21 @@ export const projectMembersResponse = {
       __typename: 'Project',
       users: {
         nodes: [
+          // Remove nulls https://gitlab.com/gitlab-org/gitlab/-/issues/329750
+          null,
+          null,
+          {
+            user: {
+              id: 'gid://gitlab/User/1',
+              avatarUrl:
+                'https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon',
+              name: 'Administrator',
+              username: 'root',
+              webUrl: '/root',
+              status: null,
+            },
+          },
+          // This is a duplicated entry https://gitlab.com/gitlab-org/gitlab/-/issues/327822
           {
             user: {
               id: 'gid://gitlab/User/1',
@@ -454,6 +469,16 @@ export const participantsQueryResponse = {
         iid: '1',
         participants: {
           nodes: [
+            {
+              id: 'gid://gitlab/User/1',
+              avatarUrl:
+                'https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon',
+              name: 'Administrator',
+              username: 'root',
+              webUrl: '/root',
+              status: null,
+            },
+            // this is a duplicated entry https://gitlab.com/gitlab-org/gitlab/-/issues/327822
             {
               id: 'gid://gitlab/User/1',
               avatarUrl: