diff --git a/packages/db/src/query/compiler/joins.ts b/packages/db/src/query/compiler/joins.ts
index afa5276b1..703d4c94a 100644
--- a/packages/db/src/query/compiler/joins.ts
+++ b/packages/db/src/query/compiler/joins.ts
@@ -297,8 +297,10 @@ function processJoin(
             return
           }
 
-          // Request filtered snapshot from lazy collection for matching join keys
-          const joinKeys = data.getInner().map(([[joinKey]]) => joinKey)
+          // Request filtered snapshot from lazy collection for unique matching join keys
+          const joinKeys = [
+            ...new Set(data.getInner().map(([[joinKey]]) => joinKey)),
+          ]
           const lazyJoinRef = new PropRef(followRefResult.path)
           const loaded = lazySourceSubscription.requestSnapshot({
             where: inArray(lazyJoinRef, joinKeys),
diff --git a/packages/db/tests/query/compiler/join-key-deduplication.test.ts b/packages/db/tests/query/compiler/join-key-deduplication.test.ts
new file mode 100644
index 000000000..22f717c42
--- /dev/null
+++ b/packages/db/tests/query/compiler/join-key-deduplication.test.ts
@@ -0,0 +1,202 @@
+import { describe, expect, it } from 'vitest'
+import { createCollection } from '../../../src/collection/index.js'
+import { createLiveQueryCollection } from '../../../src/query/live-query-collection.js'
+import { eq } from '../../../src/query/builder/functions.js'
+import type { LoadSubsetOptions } from '../../../src/types.js'
+
+interface User {
+  id: number
+  name: string
+}
+
+interface Post {
+  id: number
+  userId: number
+  title: string
+}
+
+describe(`Join Key Deduplication`, () => {
+  it(`should deduplicate join keys in the 'in' condition sent to loadSubset`, async () => {
+    const loadSubsetCalls: Array<LoadSubsetOptions> = []
+
+    const usersCollection = createCollection<User, number>({
+      id: `users`,
+      getKey: (item) => item.id,
+      syncMode: `on-demand`,
+      startSync: true,
+      sync: {
+        sync: ({ markReady }) => {
+          markReady()
+          return {
+            loadSubset: (options: LoadSubsetOptions) => {
+              loadSubsetCalls.push(options)
+              return Promise.resolve()
+            },
+          }
+        },
+      },
+    })
+
+    const postsCollection = createCollection<Post, number>({
+      id: `posts`,
+      getKey: (item) => item.id,
+      startSync: true,
+      sync: {
+        sync: ({ begin, write, commit, markReady }) => {
+          begin()
+          write({ type: `insert`, value: { id: 1, userId: 1, title: `Post 1 by User 1` } })
+          write({ type: `insert`, value: { id: 2, userId: 1, title: `Post 2 by User 1` } })
+          write({ type: `insert`, value: { id: 3, userId: 1, title: `Post 3 by User 1` } })
+          write({ type: `insert`, value: { id: 4, userId: 2, title: `Post 1 by User 2` } })
+          write({ type: `insert`, value: { id: 5, userId: 2, title: `Post 2 by User 2` } })
+          commit()
+          markReady()
+        },
+      },
+    })
+
+    await usersCollection.stateWhenReady()
+    await postsCollection.stateWhenReady()
+
+    usersCollection.createIndex((row) => row.id)
+
+    const query = createLiveQueryCollection({
+      query: (q) =>
+        q
+          .from({ post: postsCollection })
+          .leftJoin({ user: usersCollection }, ({ post, user }) =>
+            eq(post.userId, user.id),
+          )
+          .select(({ post, user }) => ({
+            id: post.id,
+            postTitle: post.title,
+            userName: user?.name,
+          })),
+      startSync: true,
+    })
+
+    await new Promise((resolve) => setTimeout(resolve, 100))
+
+    expect(loadSubsetCalls.length).toBeGreaterThan(0)
+
+    const inArrayCall = loadSubsetCalls.find(
+      (call) =>
+        call.where?.type === `func` && (call.where as any).name === `in`,
+    )
+
+    expect(inArrayCall).toBeDefined()
+
+    const whereClause = inArrayCall!.where as any
+    const inArrayArg = whereClause.args[1]
+    expect(inArrayArg.type).toBe(`val`)
+    const inArrayValues = inArrayArg.value as Array<number>
+
+    const sortedValues = [...inArrayValues].sort((a, b) => a - b)
+    expect(sortedValues).toEqual([1, 2])
+
+    const uniqueValues = [...new Set(inArrayValues)]
+    expect(inArrayValues.length).toBe(uniqueValues.length)
+
+    await query.cleanup()
+    await usersCollection.cleanup()
+    await postsCollection.cleanup()
+  })
+
+  it(`should handle join keys with string values without duplicates`, async () => {
+    interface Category {
+      slug: string
+      name: string
+    }
+
+    interface Product {
+      id: number
+      categorySlug: string
+      name: string
+    }
+
+    const loadSubsetCalls: Array<LoadSubsetOptions> = []
+
+    const categoriesCollection = createCollection<Category, string>({
+      id: `categories`,
+      getKey: (item) => item.slug,
+      syncMode: `on-demand`,
+      startSync: true,
+      sync: {
+        sync: ({ markReady }) => {
+          markReady()
+          return {
+            loadSubset: (options: LoadSubsetOptions) => {
+              loadSubsetCalls.push(options)
+              return Promise.resolve()
+            },
+          }
+        },
+      },
+    })
+
+    const productsCollection = createCollection<Product, number>({
+      id: `products`,
+      getKey: (item) => item.id,
+      startSync: true,
+      sync: {
+        sync: ({ begin, write, commit, markReady }) => {
+          begin()
+          write({ type: `insert`, value: { id: 1, categorySlug: `electronics`, name: `Phone` } })
+          write({ type: `insert`, value: { id: 2, categorySlug: `electronics`, name: `Laptop` } })
+          write({ type: `insert`, value: { id: 3, categorySlug: `electronics`, name: `Tablet` } })
+          write({ type: `insert`, value: { id: 4, categorySlug: `clothing`, name: `Shirt` } })
+          write({ type: `insert`, value: { id: 5, categorySlug: `clothing`, name: `Pants` } })
+          write({ type: `insert`, value: { id: 6, categorySlug: `books`, name: `Novel` } })
+          commit()
+          markReady()
+        },
+      },
+    })
+
+    await categoriesCollection.stateWhenReady()
+    await productsCollection.stateWhenReady()
+
+    categoriesCollection.createIndex((row) => row.slug)
+
+    const query = createLiveQueryCollection({
+      query: (q) =>
+        q
+          .from({ product: productsCollection })
+          .leftJoin({ category: categoriesCollection }, ({ product, category }) =>
+            eq(product.categorySlug, category.slug),
+          )
+          .select(({ product, category }) => ({
+            id: product.id,
+            productName: product.name,
+            categoryName: category?.name,
+          })),
+      startSync: true,
+    })
+
+    await new Promise((resolve) => setTimeout(resolve, 100))
+
+    expect(loadSubsetCalls.length).toBeGreaterThan(0)
+
+    const inArrayCall = loadSubsetCalls.find(
+      (call) =>
+        call.where?.type === `func` && (call.where as any).name === `in`,
+    )
+
+    expect(inArrayCall).toBeDefined()
+
+    const whereClause = inArrayCall!.where as any
+    const inArrayArg = whereClause.args[1]
+    expect(inArrayArg.type).toBe(`val`)
+    const inArrayValues = inArrayArg.value as Array<string>
+
+    const sortedValues = [...inArrayValues].sort()
+    expect(sortedValues).toEqual([`books`, `clothing`, `electronics`])
+
+    const uniqueValues = [...new Set(inArrayValues)]
+    expect(inArrayValues.length).toBe(uniqueValues.length)
+
+    await query.cleanup()
+    await categoriesCollection.cleanup()
+    await productsCollection.cleanup()
+  })
+})