diff --git a/src/vs/workbench/contrib/remote/browser/tunnelView.ts b/src/vs/workbench/contrib/remote/browser/tunnelView.ts
index 41e71b02b44..9f51da76ac0 100644
--- a/src/vs/workbench/contrib/remote/browser/tunnelView.ts
+++ b/src/vs/workbench/contrib/remote/browser/tunnelView.ts
@@ -23,7 +23,7 @@ import { IconLabel } from 'vs/base/browser/ui/iconLabel/iconLabel';
 import { ActionRunner, IAction } from 'vs/base/common/actions';
 import { IMenuService, MenuId, MenuRegistry, ILocalizedString } from 'vs/platform/actions/common/actions';
 import { createAndFillInContextMenuActions, createAndFillInActionBarActions, createActionViewItem } from 'vs/platform/actions/browser/menuEntryActionViewItem';
-import { IRemoteExplorerService, TunnelModel, makeAddress, TunnelType, ITunnelItem, Tunnel, TUNNEL_VIEW_ID, parseAddress, CandidatePort, TunnelPrivacy, TunnelEditId } from 'vs/workbench/services/remote/common/remoteExplorerService';
+import { IRemoteExplorerService, TunnelModel, makeAddress, TunnelType, ITunnelItem, Tunnel, TUNNEL_VIEW_ID, parseAddress, CandidatePort, TunnelPrivacy, TunnelEditId, mapHasAddressLocalhostOrAllInterfaces } from 'vs/workbench/services/remote/common/remoteExplorerService';
 import { IClipboardService } from 'vs/platform/clipboard/common/clipboardService';
 import { INotificationService, Severity } from 'vs/platform/notification/common/notification';
 import { InputBox, MessageType } from 'vs/base/browser/ui/inputbox/inputBox';
@@ -895,6 +895,7 @@ const invalidPortString: string = nls.localize('remote.tunnelsView.portNumberVal
 const maxPortNumber: number = 65536;
 const invalidPortNumberString: string = nls.localize('remote.tunnelsView.portNumberToHigh', "Port number must be \u2265 0 and < {0}.", maxPortNumber);
 const requiresSudoString: string = nls.localize('remote.tunnelView.inlineElevationMessage', "May Require Sudo");
+const alreadyForwarded: string = nls.localize('remote.tunnelView.alreadyForwarded', "Port is already forwarded");
 
 export namespace ForwardPortAction {
 	export const INLINE_ID = 'remote.tunnel.forwardInline';
@@ -903,7 +904,7 @@ export namespace ForwardPortAction {
 	export const TREEITEM_LABEL = nls.localize('remote.tunnel.forwardItem', "Forward Port");
 	const forwardPrompt = nls.localize('remote.tunnel.forwardPrompt', "Port number or address (eg. 3000 or 10.10.10.10:2000).");
 
-	function validateInput(value: string, canElevate: boolean): { content: string, severity: Severity } | null {
+	function validateInput(remoteExplorerService: IRemoteExplorerService, value: string, canElevate: boolean): { content: string, severity: Severity } | null {
 		const parsed = parseAddress(value);
 		if (!parsed) {
 			return { content: invalidPortString, severity: Severity.Error };
@@ -911,6 +912,8 @@ export namespace ForwardPortAction {
 			return { content: invalidPortNumberString, severity: Severity.Error };
 		} else if (canElevate && isPortPrivileged(parsed.port)) {
 			return { content: requiresSudoString, severity: Severity.Info };
+		} else if (mapHasAddressLocalhostOrAllInterfaces(remoteExplorerService.tunnelModel.forwarded, parsed.host, parsed.port)) {
+			return { content: alreadyForwarded, severity: Severity.Error };
 		}
 		return null;
 	}
@@ -934,7 +937,7 @@ export namespace ForwardPortAction {
 						remoteExplorerService.forward({ host: parsed.host, port: parsed.port }, undefined, undefined, undefined, true).then(tunnel => error(notificationService, tunnel, parsed!.host, parsed!.port));
 					}
 				},
-				validationMessage: (value) => validateInput(value, tunnelService.canElevate),
+				validationMessage: (value) => validateInput(remoteExplorerService, value, tunnelService.canElevate),
 				placeholder: forwardPrompt
 			});
 		};
@@ -950,7 +953,7 @@ export namespace ForwardPortAction {
 			await viewsService.openView(TunnelPanel.ID, true);
 			const value = await quickInputService.input({
 				prompt: forwardPrompt,
-				validateInput: (value) => Promise.resolve(validateInput(value, tunnelService.canElevate))
+				validateInput: (value) => Promise.resolve(validateInput(remoteExplorerService, value, tunnelService.canElevate))
 			});
 			let parsed: { host: string, port: number } | undefined;
 			if (value && (parsed = parseAddress(value))) {