fix bind:group with duplicated values

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* Fix checkbox `bind:group` when multiple options have the same value ([#4397](https://github.com/sveltejs/svelte/issues/4397))
 * Fix `bind:this` to the value of an `{#each}` block ([#4517](https://github.com/sveltejs/svelte/issues/4517))
 * Fix reactivity when assigning to contextual `{#each}` variable ([#4574](https://github.com/sveltejs/svelte/issues/4574), [#4744](https://github.com/sveltejs/svelte/issues/4744))
 * Fix binding to contextual `{#each}` values that shadow outer names ([#4757](https://github.com/sveltejs/svelte/issues/4757))

src/compiler/compile/render_dom/wrappers/Element/Binding.ts

@@ -335,7 +335,7 @@ function get_value_from_dom(
 	if (name === 'group') {
 		const binding_group = get_binding_group(renderer, binding.node.expression.node);
 		if (type === 'checkbox') {
-			return x`@get_binding_group_value($$binding_groups[${binding_group}])`;
+			return x`@get_binding_group_value($$binding_groups[${binding_group}], this.__value, this.checked)`;
 		}
 
 		return x`this.__value`;

src/runtime/internal/dom.ts

@@ -126,12 +126,15 @@ export function xlink_attr(node, attribute, value) {
 	node.setAttributeNS('http://www.w3.org/1999/xlink', attribute, value);
 }
 
-export function get_binding_group_value(group) {
-	const value = [];
+export function get_binding_group_value(group, __value, checked) {
+	const value = new Set();
 	for (let i = 0; i < group.length; i += 1) {
-		if (group[i].checked) value.push(group[i].__value);
+		if (group[i].checked) value.add(group[i].__value);
 	}
-	return value;
+	if (!checked) {
+		value.delete(__value);
+	}
+	return Array.from(value);
 }
 
 export function to_number(value) {

test/runtime/samples/binding-input-group-duplicate-value/_config.js

@@ -0,0 +1,81 @@
+export default {
+	html: `
+		<p>Checked: </p>
+
+		<hr>
+
+		<input type='checkbox' value='a'>a<br>
+		<input type='checkbox' value='b'>b<br>
+		<input type='checkbox' value='c'>c<br>
+		<input type='checkbox' value='d'>d<br>
+
+		<hr>
+
+		<input type='checkbox' value='a'>a<br>
+		<input type='checkbox' value='b'>b<br>
+		<input type='checkbox' value='c'>c<br>
+		<input type='checkbox' value='d'>d<br>
+	`,
+
+	async test({ assert, component, target, window }) {
+		const inputs = target.querySelectorAll("input");
+		const p = target.querySelector("p");
+
+		assert.equal(inputs[0].checked, false);
+		assert.equal(inputs[1].checked, false);
+		assert.equal(inputs[2].checked, false);
+		assert.equal(inputs[3].checked, false);
+
+		assert.equal(inputs[4].checked, false);
+		assert.equal(inputs[5].checked, false);
+		assert.equal(inputs[6].checked, false);
+		assert.equal(inputs[7].checked, false);
+
+		const event = new window.Event("change");
+
+		inputs[0].checked = true;
+		await inputs[0].dispatchEvent(event);
+
+		assert.htmlEqual(p.innerHTML, `Checked: a`);
+
+		assert.equal(inputs[0].checked, true);
+		assert.equal(inputs[1].checked, false);
+		assert.equal(inputs[2].checked, false);
+		assert.equal(inputs[3].checked, false);
+
+		assert.equal(inputs[4].checked, true);
+		assert.equal(inputs[5].checked, false);
+		assert.equal(inputs[6].checked, false);
+		assert.equal(inputs[7].checked, false);
+
+		inputs[3].checked = true;
+		await inputs[3].dispatchEvent(event);
+
+		assert.htmlEqual(p.innerHTML, `Checked: a,d`);
+
+		assert.equal(inputs[0].checked, true);
+		assert.equal(inputs[1].checked, false);
+		assert.equal(inputs[2].checked, false);
+		assert.equal(inputs[3].checked, true);
+
+		assert.equal(inputs[4].checked, true);
+		assert.equal(inputs[5].checked, false);
+		assert.equal(inputs[6].checked, false);
+		assert.equal(inputs[7].checked, true);
+
+		inputs[4].checked = false;
+		await inputs[4].dispatchEvent(event);
+
+		assert.htmlEqual(p.innerHTML, `Checked: d`);
+
+		assert.equal(inputs[0].checked, false);
+		assert.equal(inputs[1].checked, false);
+		assert.equal(inputs[2].checked, false);
+		assert.equal(inputs[3].checked, true);
+
+		assert.equal(inputs[4].checked, false);
+		assert.equal(inputs[5].checked, false);
+		assert.equal(inputs[6].checked, false);
+		assert.equal(inputs[7].checked, true);
+	},
+};

test/runtime/samples/binding-input-group-duplicate-value/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let foo = [];
+</script>
+
+<p>Checked: {foo}</p>
+
+<hr>
+
+<input type='checkbox' bind:group={foo} value='a'>a<br>
+<input type='checkbox' bind:group={foo} value='b'>b<br>
+<input type='checkbox' bind:group={foo} value='c'>c<br>
+<input type='checkbox' bind:group={foo} value='d'>d<br>
+
+<hr>
+
+<input type='checkbox' bind:group={foo} value='a'>a<br>
+<input type='checkbox' bind:group={foo} value='b'>b<br>
+<input type='checkbox' bind:group={foo} value='c'>c<br>
+<input type='checkbox' bind:group={foo} value='d'>d<br>