diff --git a/packages/frontend-embed/src/components/EmReactionsViewer.vue b/packages/frontend-embed/src/components/EmReactionsViewer.vue
index f5aa6bdc3f..583748abfa 100644
--- a/packages/frontend-embed/src/components/EmReactionsViewer.vue
+++ b/packages/frontend-embed/src/components/EmReactionsViewer.vue
@@ -43,8 +43,7 @@ watch([() => props.note.reactions, () => props.maxNumber], ([newSource, maxNumbe
 	for (let i = 0; i < reactions.value.length; i++) {
 		const reaction = reactions.value[i][0];
 		if (reaction in newSource && newSource[reaction] !== 0) {
-			reactions.value[i][1] = newSource[reaction];
-			newReactions.push(reactions.value[i]);
+			newReactions.push([reaction, newSource[reaction]]);
 		}
 	}
 
diff --git a/packages/frontend/src/components/MkReactionsViewer.vue b/packages/frontend/src/components/MkReactionsViewer.vue
index bd9ef50157..08a68a9564 100644
--- a/packages/frontend/src/components/MkReactionsViewer.vue
+++ b/packages/frontend/src/components/MkReactionsViewer.vue
@@ -86,8 +86,7 @@ watch([() => props.reactions, () => props.maxNumber], ([newSource, maxNumber]) =
 	for (let i = 0; i < _reactions.value.length; i++) {
 		const reaction = _reactions.value[i][0];
 		if (reaction in newSource && newSource[reaction] !== 0) {
-			_reactions.value[i][1] = newSource[reaction];
-			newReactions.push(_reactions.value[i]);
+			newReactions.push([reaction, newSource[reaction]]);
 		}
 	}
 
diff --git a/packages/frontend/test/reactions-viewer.test.ts b/packages/frontend/test/reactions-viewer.test.ts
new file mode 100644
index 0000000000..39af1ca6fd
--- /dev/null
+++ b/packages/frontend/test/reactions-viewer.test.ts
@@ -0,0 +1,105 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { describe, test, assert } from 'vitest';
+import './init';
+
+describe('MkReactionsViewer', () => {
+	test('should handle reaction count updates without duplication', () => {
+		// Test the logic that was causing the bug
+		// Simulate the corrected watch function logic
+		
+		const initialReactions = [["👍", 5], ["❤️", 3], ["🎉", 2]];
+		const newSource = {"👍": 6, "❤️": 3, "🎉": 2, "😊": 1};
+		const maxNumber = 10;
+		
+		// Apply the corrected logic
+		let newReactions: [string, number][] = [];
+		
+		for (let i = 0; i < initialReactions.length; i++) {
+			const reaction = initialReactions[i][0];
+			if (reaction in newSource && newSource[reaction] !== 0) {
+				// Fixed: Create new array instead of modifying in place
+				newReactions.push([reaction, newSource[reaction]]);
+			}
+		}
+		
+		const newReactionsNames = newReactions.map(([x]) => x);
+		newReactions = [
+			...newReactions,
+			...Object.entries(newSource)
+				.sort(([, a], [, b]) => b - a)
+				.filter(([y], i) => i < maxNumber && !newReactionsNames.includes(y)),
+		];
+		
+		// Verify results
+		const thumbsUp = newReactions.find(([emoji]) => emoji === "👍");
+		const heart = newReactions.find(([emoji]) => emoji === "❤️");
+		const newEmoji = newReactions.find(([emoji]) => emoji === "😊");
+		
+		assert.ok(thumbsUp, "👍 reaction should exist");
+		assert.strictEqual(thumbsUp[1], 6, "👍 count should be updated to 6");
+		
+		assert.ok(heart, "❤️ reaction should exist");
+		assert.strictEqual(heart[1], 3, "❤️ count should remain 3");
+		
+		assert.ok(newEmoji, "😊 reaction should be added");
+		assert.strictEqual(newEmoji[1], 1, "😊 count should be 1");
+		
+		// Verify original array is unchanged
+		assert.strictEqual(initialReactions[0][1], 5, "Original reactions should not be modified");
+	});
+
+	test('should maintain correct counts during multiple rapid updates', () => {
+		// Test scenario that would cause duplication in the old code
+		let reactions = [["👍", 5], ["❤️", 3]];
+		
+		// First update
+		const update1 = {"👍": 6, "❤️": 3, "🎉": 1};
+		let newReactions1: [string, number][] = [];
+		
+		for (let i = 0; i < reactions.length; i++) {
+			const reaction = reactions[i][0];
+			if (reaction in update1 && update1[reaction] !== 0) {
+				newReactions1.push([reaction, update1[reaction]]);
+			}
+		}
+		
+		// Add new reactions
+		const existingNames1 = newReactions1.map(([x]) => x);
+		newReactions1.push(...Object.entries(update1).filter(([name]) => !existingNames1.includes(name)));
+		
+		// Second update (simulating rapid updates when real-time mode is off)
+		const update2 = {"👍": 7, "❤️": 4, "🎉": 1, "😊": 2};
+		let newReactions2: [string, number][] = [];
+		
+		// Use the result from first update as input
+		for (let i = 0; i < newReactions1.length; i++) {
+			const reaction = newReactions1[i][0];
+			if (reaction in update2 && update2[reaction] !== 0) {
+				newReactions2.push([reaction, update2[reaction]]);
+			}
+		}
+		
+		// Add any new reactions from second update
+		const existingNames2 = newReactions2.map(([x]) => x);
+		newReactions2.push(...Object.entries(update2).filter(([name]) => !existingNames2.includes(name)));
+		
+		// Verify final state
+		const thumbsUp = newReactions2.find(([emoji]) => emoji === "👍");
+		const heart = newReactions2.find(([emoji]) => emoji === "❤️");
+		const party = newReactions2.find(([emoji]) => emoji === "🎉");
+		const smile = newReactions2.find(([emoji]) => emoji === "😊");
+		
+		assert.strictEqual(thumbsUp?.[1], 7, "👍 should have correct final count");
+		assert.strictEqual(heart?.[1], 4, "❤️ should have correct final count");
+		assert.strictEqual(party?.[1], 1, "🎉 should have correct final count");
+		assert.strictEqual(smile?.[1], 2, "😊 should have correct final count");
+		
+		// Verify no duplication occurred
+		const thumbsUpCount = newReactions2.filter(([emoji]) => emoji === "👍").length;
+		assert.strictEqual(thumbsUpCount, 1, "👍 should appear only once in final array");
+	});
+});
\ No newline at end of file