diff --git a/src/tui-opentui/keybindings.test.ts b/src/tui-opentui/keybindings.test.ts
index c44967776..59ef251d0 100644
--- a/src/tui-opentui/keybindings.test.ts
+++ b/src/tui-opentui/keybindings.test.ts
@@ -557,6 +557,7 @@ function attachOnNextPrompt(shell: AppShell, id: string): Promise<void> {
         name: `${id}.png`,
         contentType: "image/png",
         data: new Uint8Array([137, 80, 78, 71]),
+        contentHash: `hash-${id}`,
       },
     }
   })
diff --git a/src/tui-opentui/live-session-port.test.ts b/src/tui-opentui/live-session-port.test.ts
index a0aaf6074..1de856db5 100644
--- a/src/tui-opentui/live-session-port.test.ts
+++ b/src/tui-opentui/live-session-port.test.ts
@@ -104,6 +104,7 @@ describe("attachment passthrough", () => {
     name: "clipboard.png",
     contentType: "image/png",
     data: new Uint8Array([1]),
+    contentHash: "hash-1",
   }
 
   test("sendImmediate forwards attachments to the host send", () => {
diff --git a/src/tui-opentui/prompt-attachments.test.ts b/src/tui-opentui/prompt-attachments.test.ts
index 0c266d511..62f7360b5 100644
--- a/src/tui-opentui/prompt-attachments.test.ts
+++ b/src/tui-opentui/prompt-attachments.test.ts
@@ -12,6 +12,7 @@ function attachment(name: string): PendingImageAttachment {
     name,
     contentType: "image/png",
     data: new Uint8Array([1, 2, 3]),
+    contentHash: `hash-${name}`,
   }
 }
 
diff --git a/src/tui-opentui/prompt-features.test.ts b/src/tui-opentui/prompt-features.test.ts
index 350059262..c8dfe3e37 100644
--- a/src/tui-opentui/prompt-features.test.ts
+++ b/src/tui-opentui/prompt-features.test.ts
@@ -10,6 +10,7 @@ import { withTestRenderer, type Harness } from "./harness"
 import {
   acceptOverlaySelection,
   attachClipboardImage,
+  clearPendingAttachments,
   createAppShell,
   moveOverlaySelection,
   noticeText,
@@ -27,6 +28,25 @@ const CLIP: PendingImageAttachment = {
   name: "clipboard.png",
   contentType: "image/png",
   data: new Uint8Array([137, 80, 78, 71]),
+  contentHash: "hash-a",
+}
+
+// A second read of the same clipboard content: distinct id/name/timestamp
+// (as a real re-paste would produce) but identical decoded bytes and hash.
+const CLIP_SAME_CONTENT: PendingImageAttachment = {
+  id: "clip-2",
+  name: "clipboard-later.png",
+  contentType: "image/png",
+  data: new Uint8Array([137, 80, 78, 71]),
+  contentHash: "hash-a",
+}
+
+const CLIP_OTHER: PendingImageAttachment = {
+  id: "clip-3",
+  name: "clipboard-other.png",
+  contentType: "image/png",
+  data: new Uint8Array([1, 2, 3, 4]),
+  contentHash: "hash-b",
 }
 
 function withShell(
@@ -140,6 +160,48 @@ describe("image attachments", () => {
     })
   }
 
+  test("re-pasting the same clipboard content does not attach a second copy", async () => {
+    await withShell(async (shell) => {
+      let calls = 0
+      setPromptImageSource(shell, async () => {
+        calls += 1
+        return { ok: true, attachment: calls === 1 ? CLIP : CLIP_SAME_CONTENT }
+      })
+      expect(await attachClipboardImage(shell)).toBe(true)
+      expect(await attachClipboardImage(shell)).toBe(false)
+      expect(shell.pendingAttachments).toHaveLength(1)
+      expect(shell.pendingAttachments[0]?.id).toBe("clip-1")
+      // Names the attachment already sitting in the pending set (CLIP), not
+      // the rejected paste (CLIP_SAME_CONTENT) -- the operator never saw the
+      // rejected paste's filename, so naming it would read as a bug.
+      expect(shell.statusFlash).toContain(`${CLIP.name} is already attached`)
+      expect(shell.statusFlash).not.toContain(CLIP_SAME_CONTENT.name)
+    })
+  })
+
+  test("a genuinely different image still attaches alongside the first", async () => {
+    await withShell(async (shell) => {
+      let calls = 0
+      setPromptImageSource(shell, async () => {
+        calls += 1
+        return { ok: true, attachment: calls === 1 ? CLIP : CLIP_OTHER }
+      })
+      expect(await attachClipboardImage(shell)).toBe(true)
+      expect(await attachClipboardImage(shell)).toBe(true)
+      expect(shell.pendingAttachments).toHaveLength(2)
+    })
+  })
+
+  test("removing all attachments and re-pasting the same content re-attaches it", async () => {
+    await withShell(async (shell) => {
+      setPromptImageSource(shell, async () => ({ ok: true, attachment: CLIP }))
+      expect(await attachClipboardImage(shell)).toBe(true)
+      clearPendingAttachments(shell)
+      expect(await attachClipboardImage(shell)).toBe(true)
+      expect(shell.pendingAttachments).toHaveLength(1)
+    })
+  })
+
   test("submit hands pending attachments to the bridge and clears them", async () => {
     await withShell(async (shell) => {
       const seen: Array<readonly PendingImageAttachment[] | undefined> = []
diff --git a/src/tui-opentui/shell.ts b/src/tui-opentui/shell.ts
index 47efdee0d..30077c418 100644
--- a/src/tui-opentui/shell.ts
+++ b/src/tui-opentui/shell.ts
@@ -932,6 +932,13 @@ export async function attachClipboardImage(shell: AppShell): Promise<boolean> {
     setStatusFlash(shell, `image attach failed: ${result.reason}`)
     return false
   }
+  const duplicate = shell.pendingAttachments.find(
+    (attachment) => attachment.contentHash === result.attachment.contentHash,
+  )
+  if (duplicate !== undefined) {
+    setStatusFlash(shell, `${duplicate.name} is already attached`)
+    return false
+  }
   addPendingAttachment(shell, result.attachment)
   setStatusFlash(shell, `attached ${result.attachment.name}`)
   return true
diff --git a/src/tui/image-attachments.test.ts b/src/tui/image-attachments.test.ts
index e0c1500a8..2b7281b7f 100644
--- a/src/tui/image-attachments.test.ts
+++ b/src/tui/image-attachments.test.ts
@@ -1,11 +1,14 @@
 import { describe, expect, test } from "bun:test";
 import { deflateSync } from "node:zlib";
 import { unlink } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import {
   extractPastedImagePaths,
   findImagePathMentions,
   imageMimeTypeForPath,
   capImageForIngestion,
+  imageAttachmentFromPath,
   MAX_IMAGE_DIMENSION,
 } from "./image-attachments.js";
 
@@ -150,4 +153,31 @@ describe("image attachment helpers", () => {
     expect(result.data).toBe(large);
     expect(result.contentType).toBe("image/png");
   });
+
+  // The dedupe fix (see shell.ts attachClipboardImage) rests entirely on this:
+  // two ingests of identical source bytes must hash identically even though
+  // capImageForIngestion re-encodes oversized images through `sips`, whose
+  // JPEG output is not byte-stable across runs. Sizing the fixture above
+  // DOWNSCALE_THRESHOLD_BYTES (300 KB) exercises that re-encode path -- a
+  // small fixture would pass even if the hash were taken after capping.
+  test("hashes identical source bytes the same regardless of filename, even through the sips recompression path", async () => {
+    const bytes = buildTestPng(1200, 1200);
+    expect(bytes.byteLength).toBeGreaterThan(300 * 1024);
+
+    const pathA = join(tmpdir(), `corbits-hash-test-a-${process.pid}-${Date.now()}.png`);
+    const pathB = join(tmpdir(), `corbits-hash-test-b-${process.pid}-${Date.now()}.png`);
+    await Bun.write(pathA, bytes);
+    await Bun.write(pathB, bytes);
+    try {
+      const resultA = await imageAttachmentFromPath(pathA);
+      const resultB = await imageAttachmentFromPath(pathB);
+      if (!resultA.ok || !resultB.ok) throw new Error("expected both ingests to succeed");
+
+      expect(resultA.attachment.contentHash).toBe(resultB.attachment.contentHash);
+      expect(resultA.attachment.id).not.toBe(resultB.attachment.id);
+    } finally {
+      await unlink(pathA).catch(() => undefined);
+      await unlink(pathB).catch(() => undefined);
+    }
+  });
 });
diff --git a/src/tui/image-attachments.ts b/src/tui/image-attachments.ts
index ea3b56036..bec5f28f4 100644
--- a/src/tui/image-attachments.ts
+++ b/src/tui/image-attachments.ts
@@ -27,6 +27,8 @@ const IMAGE_MIME_BY_EXT: Readonly<Record<string, string>> = {
 export type PendingImageAttachment = MessageAttachment & {
   id: string;
   path?: string;
+  /** SHA-256 of the source image file's bytes, used to dedupe repeat pastes. */
+  contentHash: string;
 };
 
 export type AttachImageResult =
@@ -90,6 +92,10 @@ export async function imageAttachmentFromPath(path: string): Promise<AttachImage
     return { ok: false, reason: `image is too large; max ${formatBytes(MAX_IMAGE_ATTACHMENT_BYTES)}` };
   }
   const raw = await readFile(path);
+  // Hash the source bytes, not the (lossy, non-deterministic) capped output --
+  // two ingests of the same clipboard content must hash identically even if
+  // downscaling recompresses them differently.
+  const contentHash = await hashImageBytes(raw);
   const capped = await capImageForIngestion(raw, mimeType);
   return {
     ok: true,
@@ -99,10 +105,20 @@ export async function imageAttachmentFromPath(path: string): Promise<AttachImage
       contentType: capped.contentType,
       data: capped.data,
       path,
+      contentHash,
     },
   };
 }
 
+/** SHA-256 of the source image file's bytes, used to identify identical pastes regardless of filename or timing. */
+async function hashImageBytes(bytes: Buffer): Promise<string> {
+  // Buffer's type parameter is the looser ArrayBufferLike (it may back onto a
+  // pooled allocation), but readFile never actually hands back a
+  // SharedArrayBuffer-backed view, so this is a type-only cast, not a copy.
+  const digest = await crypto.subtle.digest("SHA-256", bytes as BufferSource);
+  return [...new Uint8Array(digest)].map((b) => b.toString(16).padStart(2, "0")).join("");
+}
+
 /**
  * Downscale/recompress an image before it enters a turn. Only shells out to
  * `sips` (macOS) when the source exceeds `DOWNSCALE_THRESHOLD_BYTES` --
diff --git a/src/tui/submit-handler.test.ts b/src/tui/submit-handler.test.ts
index 38ed0951e..7ce181be9 100644
--- a/src/tui/submit-handler.test.ts
+++ b/src/tui/submit-handler.test.ts
@@ -101,6 +101,7 @@ describe("image attachment submits", () => {
     name: "clipboard.png",
     contentType: "image/png",
     data: new Uint8Array([1, 2, 3]),
+    contentHash: "hash-1",
   };
 
   function attachmentHarness() {