diff --git a/server/src/__tests__/invite-accept-existing-member.test.ts b/server/src/__tests__/invite-accept-existing-member.test.ts
new file mode 100644
index 00000000000..913fcdbf32e
--- /dev/null
+++ b/server/src/__tests__/invite-accept-existing-member.test.ts
@@ -0,0 +1,126 @@
+import express from "express";
+import request from "supertest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { accessRoutes } from "../routes/access.js";
+import { errorHandler } from "../middleware/index.js";
+
+vi.mock("../services/index.js", () => ({
+  accessService: () => ({
+    isInstanceAdmin: vi.fn(),
+    canUser: vi.fn(),
+    hasPermission: vi.fn(),
+  }),
+  agentService: () => ({
+    getById: vi.fn(),
+  }),
+  boardAuthService: () => ({
+    createChallenge: vi.fn(),
+    resolveBoardAccess: vi.fn(),
+    assertCurrentBoardKey: vi.fn(),
+    revokeBoardApiKey: vi.fn(),
+  }),
+  deduplicateAgentName: vi.fn(),
+  logActivity: vi.fn(),
+  notifyHireApproved: vi.fn(),
+}));
+
+function createDbStub() {
+  const updateMock = vi.fn();
+  const invite = {
+    id: "invite-1",
+    companyId: "company-1",
+    inviteType: "company_join",
+    allowedJoinTypes: "human",
+    tokenHash: "hash",
+    defaultsPayload: { humanRole: "viewer" },
+    expiresAt: new Date("2027-03-10T00:00:00.000Z"),
+    invitedByUserId: "user-1",
+    revokedAt: null,
+    acceptedAt: null,
+    createdAt: new Date("2026-03-07T00:00:00.000Z"),
+    updatedAt: new Date("2026-03-07T00:00:00.000Z"),
+  };
+
+  const db = {
+    select() {
+      return {
+        from() {
+          return {
+            where() {
+              return Promise.resolve([invite]);
+            },
+          };
+        },
+      };
+    },
+    update(...args: unknown[]) {
+      updateMock(...args);
+      return {
+        set() {
+          return {
+            where() {
+              return {
+                returning() {
+                  return Promise.resolve([]);
+                },
+              };
+            },
+          };
+        },
+      };
+    },
+  };
+
+  return { db, updateMock };
+}
+
+function createApp(db: Record<string, unknown>) {
+  const app = express();
+  app.use(express.json());
+  app.use((req, _res, next) => {
+    (req as any).actor = {
+      type: "board",
+      source: "session",
+      userId: "user-1",
+      companyIds: ["company-1"],
+      memberships: [
+        {
+          companyId: "company-1",
+          membershipRole: "owner",
+          status: "active",
+        },
+      ],
+    };
+    next();
+  });
+  app.use(
+    "/api",
+    accessRoutes(db as any, {
+      deploymentMode: "authenticated",
+      deploymentExposure: "private",
+      bindHost: "127.0.0.1",
+      allowedHostnames: [],
+    }),
+  );
+  app.use(errorHandler);
+  return app;
+}
+
+describe("POST /invites/:token/accept", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("does not consume a human invite when the signed-in user is already a company member", async () => {
+    const { db, updateMock } = createDbStub();
+    const app = createApp(db);
+
+    const res = await request(app)
+      .post("/api/invites/pcp_invite_test/accept")
+      .send({ requestType: "human" });
+
+    expect(res.status).toBe(409);
+    expect(res.body.error).toBe("You already belong to this company");
+    expect(updateMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/server/src/routes/access.ts b/server/src/routes/access.ts
index 444ab905c90..d4af585997d 100644
--- a/server/src/routes/access.ts
+++ b/server/src/routes/access.ts
@@ -918,6 +918,18 @@ function toInviteSummaryResponse(
   };
 }
 
+function actorHasActiveUserMembership(req: Request, companyId: string) {
+  return (
+    req.actor.type === "board" &&
+    typeof req.actor.userId === "string" &&
+    Array.isArray(req.actor.memberships) &&
+    req.actor.memberships.some(
+      (membership) =>
+        membership.companyId === companyId && membership.status === "active",
+    )
+  );
+}
+
 async function loadUsersById(db: Db, userIds: string[]) {
   if (userIds.length === 0) return new Map<string, ReturnType<typeof toUserProfile>>();
   const rows = await db
@@ -2697,6 +2709,12 @@ export function accessRoutes(
       ) {
         throw unauthorized("Authenticated user is required");
       }
+      if (
+        requestType === "human" &&
+        actorHasActiveUserMembership(req, companyId)
+      ) {
+        throw conflict("You already belong to this company");
+      }
       if (requestType === "agent" && !req.body.agentName) {
         if (
           !inviteAlreadyAccepted ||
diff --git a/ui/src/pages/InviteLanding.test.tsx b/ui/src/pages/InviteLanding.test.tsx
index a8802532615..18f272917b6 100644
--- a/ui/src/pages/InviteLanding.test.tsx
+++ b/ui/src/pages/InviteLanding.test.tsx
@@ -13,6 +13,7 @@ const getSessionMock = vi.hoisted(() => vi.fn());
 const signInEmailMock = vi.hoisted(() => vi.fn());
 const signUpEmailMock = vi.hoisted(() => vi.fn());
 const healthGetMock = vi.hoisted(() => vi.fn());
+const listCompaniesMock = vi.hoisted(() => vi.fn());
 
 vi.mock("../api/access", () => ({
   accessApi: {
@@ -35,6 +36,12 @@ vi.mock("../api/health", () => ({
   },
 }));
 
+vi.mock("../api/companies", () => ({
+  companiesApi: {
+    list: () => listCompaniesMock(),
+  },
+}));
+
 vi.mock("@/context/CompanyContext", () => ({
   useCompany: () => ({
     selectedCompany: null,
@@ -98,6 +105,7 @@ describe("InviteLandingPage", () => {
       status: "ok",
       deploymentMode: "authenticated",
     });
+    listCompaniesMock.mockResolvedValue([]);
     getSessionMock.mockResolvedValue(null);
     signInEmailMock.mockResolvedValue(undefined);
     signUpEmailMock.mockResolvedValue(undefined);
@@ -225,4 +233,57 @@ describe("InviteLandingPage", () => {
       root.unmount();
     });
   });
+
+  it("waits for the membership check before showing invite acceptance to signed-in users", async () => {
+    let resolveCompanies: ((value: Array<{ id: string; name: string }>) => void) | null = null;
+    listCompaniesMock.mockImplementation(
+      () =>
+        new Promise<Array<{ id: string; name: string }>>((resolve) => {
+          resolveCompanies = resolve;
+        }),
+    );
+    getSessionMock.mockResolvedValue({
+      session: { id: "session-1", userId: "user-1" },
+      user: {
+        id: "user-1",
+        name: "Jane Example",
+        email: "jane@example.com",
+        image: null,
+      },
+    });
+
+    const root = createRoot(container);
+    const queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } },
+    });
+
+    await act(async () => {
+      root.render(
+        <MemoryRouter initialEntries={["/invite/pcp_invite_test"]}>
+          <QueryClientProvider client={queryClient}>
+            <Routes>
+              <Route path="/invite/:token" element={<InviteLandingPage />} />
+            </Routes>
+          </QueryClientProvider>
+        </MemoryRouter>,
+      );
+    });
+    await flushReact();
+
+    expect(container.textContent).toContain("Checking your access...");
+    expect(container.textContent).not.toContain("Join company");
+    expect(acceptInviteMock).not.toHaveBeenCalled();
+
+    await act(async () => {
+      resolveCompanies?.([{ id: "company-1", name: "Acme Robotics" }]);
+    });
+    await flushReact();
+    await flushReact();
+
+    expect(acceptInviteMock).not.toHaveBeenCalled();
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
 });
diff --git a/ui/src/pages/InviteLanding.tsx b/ui/src/pages/InviteLanding.tsx
index c204aa20744..14ccf45a767 100644
--- a/ui/src/pages/InviteLanding.tsx
+++ b/ui/src/pages/InviteLanding.tsx
@@ -93,6 +93,15 @@ export function InviteLandingPage() {
   }, [companiesQuery.data, inviteQuery.data, token, navigate]);
 
   const invite = inviteQuery.data;
+  const isCheckingExistingMembership =
+    Boolean(sessionQuery.data) &&
+    Boolean(invite?.companyId) &&
+    companiesQuery.isLoading;
+  const isCurrentMember =
+    Boolean(invite?.companyId) &&
+    Boolean(
+      companiesQuery.data?.some((company) => company.id === invite?.companyId),
+    );
   const companyName = invite?.companyName?.trim() || null;
   const companyDisplayName = companyName || "this Paperclip company";
   const companyLogoUrl = invite?.companyLogoUrl?.trim() || null;
@@ -116,6 +125,12 @@ export function InviteLandingPage() {
   const acceptMutation = useMutation({
     mutationFn: async () => {
       if (!invite) throw new Error("Invite not found");
+      if (isCheckingExistingMembership) {
+        throw new Error("Checking your company access. Try again in a moment.");
+      }
+      if (isCurrentMember) {
+        throw new Error("This account already belongs to the company.");
+      }
       if (invite.inviteType === "bootstrap_ceo" || invite.allowedJoinTypes !== "agent") {
         return accessApi.acceptInvite(token, { requestType: "human" });
       }
@@ -178,6 +193,10 @@ export function InviteLandingPage() {
     return <div className="mx-auto max-w-xl py-10 text-sm text-muted-foreground">Loading invite...</div>;
   }
 
+  if (isCheckingExistingMembership) {
+    return <div className="mx-auto max-w-xl py-10 text-sm text-muted-foreground">Checking your access...</div>;
+  }
+
   if (inviteQuery.error || !invite) {
     return (
       <div className="mx-auto max-w-xl py-10">