add dev mode check for keyed each duplicate keys (#4301)

src/compiler/compile/render_dom/wrappers/EachBlock.ts

@@ -374,6 +374,7 @@ export default class EachBlockWrapper extends Wrapper {
 		block.chunks.init.push(b`
 			const ${get_key} = #ctx => ${this.node.key.manipulate(block)};
 
+			${this.renderer.options.dev && b`@validate_each_keys(#ctx, ${this.vars.each_block_value}, ${this.vars.get_each_context}, ${get_key});`}
 			for (let #i = 0; #i < ${data_length}; #i += 1) {
 				let child_ctx = ${this.vars.get_each_context}(#ctx, ${this.vars.each_block_value}, #i);
 				let key = ${get_key}(child_ctx);
@@ -416,6 +417,7 @@ export default class EachBlockWrapper extends Wrapper {
 
 			${this.block.has_outros && b`@group_outros();`}
 			${this.node.has_animation && b`for (let #i = 0; #i < ${view_length}; #i += 1) ${iterations}[#i].r();`}
+			${this.renderer.options.dev && b`@validate_each_keys(#ctx, ${this.vars.each_block_value}, ${this.vars.get_each_context}, ${get_key});`}
 			${iterations} = @update_keyed_each(${iterations}, #dirty, ${get_key}, ${dynamic ? 1 : 0}, #ctx, ${this.vars.each_block_value}, ${lookup}, ${update_mount_node}, ${destroy}, ${create_each_block}, ${update_anchor_node}, ${this.vars.get_each_context});
 			${this.node.has_animation && b`for (let #i = 0; #i < ${view_length}; #i += 1) ${iterations}[#i].a();`}
 			${this.block.has_outros && b`@check_outros();`}

src/runtime/internal/keyed_each.ts

@@ -108,9 +108,13 @@ export function update_keyed_each(old_blocks, dirty, get_key, dynamic, ctx, list
 	return new_blocks;
 }
 
-export function measure(blocks) {
+export function validate_each_keys(ctx, list, get_context, get_key) {
-	const rects = {};
+	const keys = new Set();
-	let i = blocks.length;
+	for (let i = 0; i < list.length; i++) {
-	while (i--) rects[blocks[i].key] = blocks[i].node.getBoundingClientRect();
+		const key = get_key(get_context(ctx, list, i));
-	return rects;
+		if (keys.has(key)) {
+			throw new Error(`Cannot have duplicate keys in a keyed each`);
+		}
+		keys.add(key);
+	}
 }