diff --git a/src/docker-hosts.ts b/src/docker-hosts.ts
index d83ef866..6a0d705b 100644
--- a/src/docker-hosts.ts
+++ b/src/docker-hosts.ts
@@ -293,6 +293,64 @@ export function toSessionDocker(host: DockerHost, dockerCase: DockerCase): Sessi
   return session;
 }
 
+/**
+ * Which existing case, if any, blocks adopting `container` at `containerWorkdir`.
+ *
+ * One container may back SEVERAL adopted cases, each pointing at a different
+ * directory inside it — that is the whole reason to adopt the same container
+ * twice, and it is safe because the in-container tmux session is named per
+ * SESSION (`dockerTmuxSessionName`, `codeman-dkr-<id8>`) and not per case, so a
+ * session teardown kills exactly one session and its siblings on the shared
+ * in-container tmux server are untouched. Nothing else reaches an adopted
+ * container's lifecycle either: stop/remove throw at the builder, recreate
+ * refuses `owned === false`, and the orphan reaper filters on the
+ * `codeman.managed=1` label that only Codeman-created containers carry.
+ *
+ * So the conflicts that remain are NOT about the tmux server:
+ *  - `owned-case`  the container backs a case Codeman CREATED, whose lifecycle
+ *                  it owns; a recreate or delete there would destroy the
+ *                  adopted case's container out from under it.
+ *  - `other-owner` already adopted by a different user. Adoption hands out a
+ *                  shell inside someone else's container, so it stays scoped.
+ *  - `duplicate`   same container AND same directory: the second case would
+ *                  behave identically to the first, so name the first instead
+ *                  of silently creating a twin. A DIFFERENT directory is the
+ *                  supported case and returns null.
+ */
+export type AdoptContainerConflict =
+  | { kind: 'owned-case'; caseName: string }
+  | { kind: 'other-owner'; caseName: string }
+  | { kind: 'duplicate'; caseName: string }
+  | null;
+
+export function classifyAdoptContainerConflict(params: {
+  container: string;
+  /** Directory inside the container this adoption targets (already defaulted). */
+  containerWorkdir: string;
+  existing: ReadonlyArray<
+    Pick<DockerCase, 'name' | 'container' | 'containerWorkdir' | 'hostWorkspacePath' | 'owned' | 'owner'>
+  >;
+  /** Owner visibility test (canAccessOwned bound to the caller). */
+  canAccess: (owner?: string) => boolean;
+}): AdoptContainerConflict {
+  const { container, containerWorkdir, existing, canAccess } = params;
+  const sharing = existing.filter((item) => (item.container ?? dockerContainerName(item.name)) === container);
+  if (sharing.length === 0) return null;
+
+  // `owned` is optional and an ABSENT flag means owned (legacy cases predate the
+  // field), so this must test `!== false` rather than truthiness.
+  const owned = sharing.find((item) => item.owned !== false);
+  if (owned) return { kind: 'owned-case', caseName: owned.name };
+
+  const foreign = sharing.find((item) => !canAccess(item.owner));
+  if (foreign) return { kind: 'other-owner', caseName: foreign.name };
+
+  const twin = sharing.find((item) => (item.containerWorkdir ?? item.hostWorkspacePath) === containerWorkdir);
+  if (twin) return { kind: 'duplicate', caseName: twin.name };
+
+  return null;
+}
+
 /**
  * An ADOPTED container is one the user built and runs themselves. Codeman may
  * only exec into it; it must never create, start, stop, restart or remove it.
diff --git a/src/web/routes/case-routes.ts b/src/web/routes/case-routes.ts
index d202c8bc..31551df0 100644
--- a/src/web/routes/case-routes.ts
+++ b/src/web/routes/case-routes.ts
@@ -79,6 +79,7 @@ import {
   dockerContainerName,
   dockerDisplayPath,
   probeAdoptableContainer,
+  classifyAdoptContainerConflict,
   listDockerContainers,
   browseInContainer,
   dockerAdoptProbeModes,
@@ -906,12 +907,35 @@ export function registerCaseRoutes(app: FastifyInstance, ctx: EventPort & Config
       ) {
         return createErrorResponse(ApiErrorCode.ALREADY_EXISTS, 'Case already exists');
       }
-      // Two cases must never share one adopted container: session close kills the
-      // in-container tmux by session id, but a shared adoption would let one case's
-      // teardown and another's launch race over the same tmux server.
+      // One container may back SEVERAL adopted cases, each pointing at a different
+      // directory inside it. What still blocks it, and why, lives in
+      // classifyAdoptContainerConflict — note that none of it is about the shared
+      // in-container tmux server, which is safe precisely because sessions there
+      // are named per SESSION id (`codeman-dkr-<id8>`), never per case.
       const container = dockerCase.container;
-      if (dockerCases.some((item) => (item.container ?? dockerContainerName(item.name)) === container)) {
-        return createErrorResponse(ApiErrorCode.ALREADY_EXISTS, `Container "${container}" is already linked to a case`);
+      const conflict = classifyAdoptContainerConflict({
+        container,
+        containerWorkdir: dockerCase.containerWorkdir ?? dockerCase.hostWorkspacePath,
+        existing: dockerCases,
+        canAccess: (owner) => canAccessOwned(getAuthUser(req), owner),
+      });
+      if (conflict?.kind === 'owned-case') {
+        return createErrorResponse(
+          ApiErrorCode.ALREADY_EXISTS,
+          `Container "${container}" belongs to case "${conflict.caseName}", which Codeman created and whose lifecycle it manages. Adopt a container you started yourself, or open that case directly.`
+        );
+      }
+      if (conflict?.kind === 'other-owner') {
+        return createErrorResponse(
+          ApiErrorCode.FORBIDDEN,
+          `Container "${container}" is already adopted by another user.`
+        );
+      }
+      if (conflict?.kind === 'duplicate') {
+        return createErrorResponse(
+          ApiErrorCode.ALREADY_EXISTS,
+          `Case "${conflict.caseName}" already adopts "${container}" at that same directory. Point this one at another directory inside the container.`
+        );
       }
 
       if (!isWorkingDirAllowed(getAuthUser(req), dockerCase.hostWorkspacePath)) {
diff --git a/test/docker-adopted-container.test.ts b/test/docker-adopted-container.test.ts
index 967c5237..1f3c4afb 100644
--- a/test/docker-adopted-container.test.ts
+++ b/test/docker-adopted-container.test.ts
@@ -19,6 +19,8 @@ import {
   checkDockerConfigDrift,
   dockerConfigHash,
   dockerAdoptProbeModes,
+  classifyAdoptContainerConflict,
+  dockerContainerName,
 } from '../src/docker-hosts.js';
 import { enabledCliIds, getCli } from '../src/config/cli-registry/index.js';
 import {
@@ -451,3 +453,113 @@ describe('adopted container: a missing container means different things per owne
     expect(routes).toContain('...(dockerCase.owned === false ? { owned: false } : {}),');
   });
 });
+
+describe('adopted container: one container may back several cases', () => {
+  const base = {
+    type: 'docker' as const,
+    hostId: 'h1',
+    hostWorkspacePath: '/srv/work',
+  };
+  const mk = (over: Record<string, unknown>) => ({ ...base, ...over }) as never;
+  const mine = () => true;
+
+  it('allows a second adoption of the same container at a DIFFERENT directory', () => {
+    // The whole point of the feature: one container, two folders, two cases.
+    const conflict = classifyAdoptContainerConflict({
+      container: 'devbox',
+      containerWorkdir: '/app/api',
+      existing: [mk({ name: 'web', container: 'devbox', containerWorkdir: '/app/web', owned: false })],
+      canAccess: mine,
+    });
+    expect(conflict).toBeNull();
+  });
+
+  it('refuses an exact twin (same container AND same directory) and names the first case', () => {
+    const conflict = classifyAdoptContainerConflict({
+      container: 'devbox',
+      containerWorkdir: '/app/web',
+      existing: [mk({ name: 'web', container: 'devbox', containerWorkdir: '/app/web', owned: false })],
+      canAccess: mine,
+    });
+    expect(conflict).toEqual({ kind: 'duplicate', caseName: 'web' });
+  });
+
+  it('falls back to hostWorkspacePath when containerWorkdir is absent on either side', () => {
+    // containerWorkdir defaults to hostWorkspacePath, so an absent field on the
+    // stored case must compare equal to an incoming adoption that omits it too —
+    // otherwise the twin check silently stops firing for the default case.
+    const conflict = classifyAdoptContainerConflict({
+      container: 'devbox',
+      containerWorkdir: '/srv/work',
+      existing: [mk({ name: 'web', container: 'devbox', owned: false })],
+      canAccess: mine,
+    });
+    expect(conflict).toEqual({ kind: 'duplicate', caseName: 'web' });
+  });
+
+  it('still refuses a container backing a case Codeman CREATED', () => {
+    // Codeman owns that container's lifecycle: a recreate or case-delete there
+    // would destroy the adopted case's container out from under it.
+    const conflict = classifyAdoptContainerConflict({
+      container: 'codeman-case-web',
+      containerWorkdir: '/app/api',
+      existing: [mk({ name: 'web', container: 'codeman-case-web', owned: true })],
+      canAccess: mine,
+    });
+    expect(conflict).toEqual({ kind: 'owned-case', caseName: 'web' });
+  });
+
+  it('treats an ABSENT owned flag as owned, so legacy cases keep the old refusal', () => {
+    const conflict = classifyAdoptContainerConflict({
+      container: 'legacy',
+      containerWorkdir: '/app/api',
+      existing: [mk({ name: 'old', container: 'legacy' })],
+      canAccess: mine,
+    });
+    expect(conflict).toEqual({ kind: 'owned-case', caseName: 'old' });
+  });
+
+  it('derives the container name from the case name when the field is absent', () => {
+    const conflict = classifyAdoptContainerConflict({
+      container: dockerContainerName('web'),
+      containerWorkdir: '/app/api',
+      existing: [mk({ name: 'web', owned: true })],
+      canAccess: mine,
+    });
+    expect(conflict).toEqual({ kind: 'owned-case', caseName: 'web' });
+  });
+
+  it('refuses a container another user already adopted', () => {
+    const conflict = classifyAdoptContainerConflict({
+      container: 'devbox',
+      containerWorkdir: '/app/api',
+      existing: [mk({ name: 'theirs', container: 'devbox', owned: false, owner: 'bob' })],
+      canAccess: (owner) => owner === 'alice',
+    });
+    expect(conflict).toEqual({ kind: 'other-owner', caseName: 'theirs' });
+  });
+
+  it('an owned case outranks a foreign adoption, so the message names the real blocker', () => {
+    const conflict = classifyAdoptContainerConflict({
+      container: 'devbox',
+      containerWorkdir: '/app/api',
+      existing: [
+        mk({ name: 'theirs', container: 'devbox', owned: false, owner: 'bob' }),
+        mk({ name: 'built', container: 'devbox', owned: true }),
+      ],
+      canAccess: (owner) => owner === 'alice',
+    });
+    expect(conflict).toEqual({ kind: 'owned-case', caseName: 'built' });
+  });
+
+  it('leaves an unrelated container alone', () => {
+    expect(
+      classifyAdoptContainerConflict({
+        container: 'fresh',
+        containerWorkdir: '/app',
+        existing: [mk({ name: 'web', container: 'devbox', owned: false })],
+        canAccess: mine,
+      })
+    ).toBeNull();
+  });
+});