diff --git a/apps/meteor/client/components/UserAutoCompleteMultiple/UserAutoCompleteMultiple.tsx b/apps/meteor/client/components/UserAutoCompleteMultiple/UserAutoCompleteMultiple.tsx
index 413cab7f6ea0c..9b1bf73c6f9a5 100644
--- a/apps/meteor/client/components/UserAutoCompleteMultiple/UserAutoCompleteMultiple.tsx
+++ b/apps/meteor/client/components/UserAutoCompleteMultiple/UserAutoCompleteMultiple.tsx
@@ -15,6 +15,7 @@ type UserAutoCompleteMultipleProps = {
 	placeholder?: string;
 	federated?: boolean;
 	error?: string;
+	exceptions?: string[];
 } & Omit<AllHTMLAttributes<HTMLInputElement>, 'is' | 'onChange' | 'value'>;
 
 type UserAutoCompleteOptionType = {
@@ -29,7 +30,7 @@ type UserAutoCompleteOptions = {
 
 const matrixRegex = new RegExp('@(.*:.*)');
 
-const UserAutoCompleteMultiple = ({ onChange, value, placeholder, federated, ...props }: UserAutoCompleteMultipleProps): ReactElement => {
+const UserAutoCompleteMultiple = ({ onChange, value, placeholder, federated, exceptions = [], ...props }: UserAutoCompleteMultipleProps): ReactElement => {
 	const [filter, setFilter] = useState('');
 	const [selectedCache, setSelectedCache] = useState<UserAutoCompleteOptions>({});
 
@@ -37,10 +38,10 @@ const UserAutoCompleteMultiple = ({ onChange, value, placeholder, federated, ...
 	const getUsers = useEndpoint('GET', '/v1/users.autocomplete');
 
 	const { data } = useQuery({
-		queryKey: usersQueryKeys.userAutoComplete(debouncedFilter, federated ?? false),
+		queryKey: usersQueryKeys.userAutoComplete(debouncedFilter, federated ?? false, exceptions),
 
 		queryFn: async () => {
-			const users = await getUsers({ selector: JSON.stringify({ term: debouncedFilter }) });
+			const users = await getUsers({ selector: JSON.stringify({ term: debouncedFilter, exceptions }) });
 			const options = users.items.map((item): [string, UserAutoCompleteOptionType] => [item.username, item]);
 
 			// Add extra option if filter text matches `username:server`
diff --git a/apps/meteor/client/lib/queryKeys.ts b/apps/meteor/client/lib/queryKeys.ts
index 44529ad6d2543..37f5c07bb1a64 100644
--- a/apps/meteor/client/lib/queryKeys.ts
+++ b/apps/meteor/client/lib/queryKeys.ts
@@ -119,7 +119,9 @@ export const usersQueryKeys = {
 	all: ['users'] as const,
 	userInfo: ({ uid, username }: { uid?: IUser['_id']; username?: IUser['username'] }) =>
 		[...usersQueryKeys.all, 'info', { uid, username }] as const,
-	userAutoComplete: (filter: string, federated: boolean) => [...usersQueryKeys.all, 'autocomplete', filter, federated] as const,
+	userAutoComplete: (filter: string, federated: boolean, exceptions: string[] = []) =>
+		// Expects exceptions to be pre-sorted in the calling component for cache efficiency
+		[...usersQueryKeys.all, 'autocomplete', filter, federated, exceptions] as const,
 };
 
 export const teamsQueryKeys = {
diff --git a/apps/meteor/client/views/room/contextualBar/RoomMembers/AddUsers/AddUsers.tsx b/apps/meteor/client/views/room/contextualBar/RoomMembers/AddUsers/AddUsers.tsx
index 7c7151d056c5f..b57e66da9520a 100644
--- a/apps/meteor/client/views/room/contextualBar/RoomMembers/AddUsers/AddUsers.tsx
+++ b/apps/meteor/client/views/room/contextualBar/RoomMembers/AddUsers/AddUsers.tsx
@@ -11,8 +11,9 @@ import {
 	ContextualbarFooter,
 	ContextualbarDialog,
 } from '@rocket.chat/ui-client';
-import { useToastMessageDispatch, useMethod, useRoomToolbox } from '@rocket.chat/ui-contexts';
-import { useId } from 'react';
+import { useToastMessageDispatch, useMethod, useRoomToolbox, useEndpoint } from '@rocket.chat/ui-contexts';
+import { useQuery } from '@tanstack/react-query';
+import { useId, useMemo } from 'react';
 import type { ReactElement } from 'react';
 import { Controller, useForm } from 'react-hook-form';
 import { useTranslation } from 'react-i18next';
@@ -42,6 +43,34 @@ const AddUsers = ({ rid, onClickBack, reload }: AddUsersProps): ReactElement =>
 	const { closeTab } = useRoomToolbox();
 	const saveAction = useMethod('addUsersToRoom');
 
+	// Fetch existing room members to exclude them from autocomplete
+	// Note: Limited to 100 members due to API_Upper_Count_Limit setting
+	// In very large channels (>100 members), some existing members might still appear in autocomplete
+	const getRoomMembers = useEndpoint('GET', '/v1/rooms.membersOrderedByRole');
+	const { data: membersData, isError: isMembersQueryError } = useQuery({
+		queryKey: ['room-members', rid],
+		queryFn: () => getRoomMembers({ roomId: rid, offset: 0, count: 100 }),
+		// Disable query if room type is not supported
+		enabled: room.t === 'c' || room.t === 'p',
+	});
+
+	const existingMemberUsernames = useMemo(() => {
+		// If the query is disabled or failed, return empty array (no exceptions)
+		// This means existing members might appear in autocomplete, but backend will handle duplicates
+		if (isMembersQueryError || !membersData?.members) {
+			return [];
+		}
+		// Use single iteration with reduce for better performance
+		return membersData.members
+			.reduce<string[]>((acc, member) => {
+				if (member.username) {
+					acc.push(member.username);
+				}
+				return acc;
+			}, [])
+			.sort(); // Sort here for consistent query cache keys
+	}, [membersData, isMembersQueryError]);
+
 	const {
 		handleSubmit,
 		control,
@@ -84,6 +113,7 @@ const AddUsers = ({ rid, onClickBack, reload }: AddUsersProps): ReactElement =>
 									federated={isFederated}
 									placeholder={t('Choose_users')}
 									aria-describedby={`${usersFieldId}-error`}
+									exceptions={existingMemberUsernames}
 									{...field}
 								/>
 							)}
diff --git a/apps/meteor/tests/end-to-end/api/channels.ts b/apps/meteor/tests/end-to-end/api/channels.ts
index f2d348240f3cf..c0feecae17016 100644
--- a/apps/meteor/tests/end-to-end/api/channels.ts
+++ b/apps/meteor/tests/end-to-end/api/channels.ts
@@ -250,6 +250,28 @@ describe('[Channels]', () => {
 			});
 	});
 
+	it('/channels.invite should still succeed when inviting a user already in the channel', async () => {
+		const roomInfo = await getRoomInfo(channel._id);
+
+		return request
+			.post(api('channels.invite'))
+			.set(credentials)
+			.send({
+				roomId: channel._id,
+				userId: 'rocket.cat',
+			})
+			.expect('Content-Type', 'application/json')
+			.expect(200)
+			.expect((res) => {
+				expect(res.body).to.have.property('success', true);
+				expect(res.body).to.have.nested.property('channel._id');
+				expect(res.body).to.have.nested.property('channel.name', apiPublicChannelName);
+				expect(res.body).to.have.nested.property('channel.t', 'c');
+				// When adding an already-existing user, no new system message is created
+				expect(res.body).to.have.nested.property('channel.msgs', roomInfo.channel.msgs);
+			});
+	});
+
 	it('/channels.addOwner', (done) => {
 		void request
 			.post(api('channels.addOwner'))