diff --git a/.husky/pre-push b/.husky/pre-push
index 53e984fe..ed318ed8 100755
--- a/.husky/pre-push
+++ b/.husky/pre-push
@@ -1 +1,2 @@
+eval "$(fnm env)" && fnm use --silent-if-unchanged
 npm run quality
diff --git a/api/src/storages/mongo.ts b/api/src/storages/mongo.ts
index 389e3ea4..d1dfba54 100644
--- a/api/src/storages/mongo.ts
+++ b/api/src/storages/mongo.ts
@@ -395,8 +395,21 @@ class MongodbStorage implements SdStorage {
       return o.id === orga.id && (o.department || null) === (department || null)
     })
 
-    if (config.singleMembership && !userOrga && user.organizations.find(o => o.id === orga.id)) {
-      throw httpError(400, 'cet utilisateur est déjà membre de cette organisation.')
+    if (config.singleMembership && !userOrga) {
+      const existingOrga = user.organizations.find(o => o.id === orga.id)
+      if (existingOrga) {
+        // update existing membership instead of rejecting
+        userOrga = existingOrga
+        if (department) {
+          const fullDepartment = orga.departments?.find(d => d.id === department)
+          if (!fullDepartment) throw httpError(404, 'department not found')
+          userOrga.department = department
+          userOrga.departmentName = fullDepartment.name
+        } else {
+          delete userOrga.department
+          delete userOrga.departmentName
+        }
+      }
     }
 
     if (!userOrga) {
diff --git a/test-it/single-membership.ts b/test-it/single-membership.ts
new file mode 100644
index 00000000..e71ab8bb
--- /dev/null
+++ b/test-it/single-membership.ts
@@ -0,0 +1,114 @@
+import { strict as assert } from 'node:assert'
+import { it, describe, before, beforeEach, after } from 'node:test'
+import { clean, startApiServer, stopApiServer, createUser } from './utils/index.ts'
+
+process.env.STORAGE_TYPE = 'mongo'
+
+describe('singleMembership addMember', () => {
+  before(startApiServer)
+  beforeEach(async () => await clean())
+  after(stopApiServer)
+
+  it('should update department when user already belongs to org', async () => {
+    const config = (await import('../api/src/config.ts')).default
+    config.singleMembership = true
+    config.alwaysAcceptInvitation = true
+
+    try {
+      const { ax } = await createUser('owner-sm@test.com')
+      const org = (await ax.post('/api/organizations', {
+        name: 'test-sm',
+        departments: [
+          { id: 'dep1', name: 'Department 1' },
+          { id: 'dep2', name: 'Department 2' }
+        ]
+      })).data
+      ax.setOrg(org.id)
+
+      // invite a user into dep1
+      await ax.post('/api/invitations', { id: org.id, name: org.name, department: 'dep1', email: 'member-sm@test.com', role: 'user' })
+
+      let members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const member = members.find(m => m.email === 'member-sm@test.com')
+      assert.equal(member.department, 'dep1')
+      assert.equal(member.departmentName, 'Department 1')
+
+      // now re-invite the same user into dep2 — with singleMembership this should update, not reject
+      await ax.post('/api/invitations', { id: org.id, name: org.name, department: 'dep2', email: 'member-sm@test.com', role: 'user' })
+
+      members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const memberEntries = members.filter(m => m.email === 'member-sm@test.com')
+      // singleMembership: only one entry, department updated
+      assert.equal(memberEntries.length, 1)
+      assert.equal(memberEntries[0].department, 'dep2')
+      assert.equal(memberEntries[0].departmentName, 'Department 2')
+    } finally {
+      config.singleMembership = false
+      config.alwaysAcceptInvitation = false
+    }
+  })
+
+  it('should update role when user already belongs to org', async () => {
+    const config = (await import('../api/src/config.ts')).default
+    config.singleMembership = true
+    config.alwaysAcceptInvitation = true
+
+    try {
+      const { ax } = await createUser('owner-sm2@test.com')
+      const org = (await ax.post('/api/organizations', { name: 'test-sm2' })).data
+      ax.setOrg(org.id)
+
+      // invite a user as 'user'
+      await ax.post('/api/invitations', { id: org.id, name: org.name, email: 'member-sm2@test.com', role: 'user' })
+
+      let members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const member = members.find(m => m.email === 'member-sm2@test.com')
+      assert.equal(member.role, 'user')
+
+      // re-invite as 'admin' — should update existing membership
+      await ax.post('/api/invitations', { id: org.id, name: org.name, email: 'member-sm2@test.com', role: 'admin' })
+
+      members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const memberEntries = members.filter(m => m.email === 'member-sm2@test.com')
+      assert.equal(memberEntries.length, 1)
+      assert.equal(memberEntries[0].role, 'admin')
+    } finally {
+      config.singleMembership = false
+      config.alwaysAcceptInvitation = false
+    }
+  })
+
+  it('should remove department when re-added without one', async () => {
+    const config = (await import('../api/src/config.ts')).default
+    config.singleMembership = true
+    config.alwaysAcceptInvitation = true
+
+    try {
+      const { ax } = await createUser('owner-sm3@test.com')
+      const org = (await ax.post('/api/organizations', {
+        name: 'test-sm3',
+        departments: [{ id: 'dep1', name: 'Department 1' }]
+      })).data
+      ax.setOrg(org.id)
+
+      // invite a user into dep1
+      await ax.post('/api/invitations', { id: org.id, name: org.name, department: 'dep1', email: 'member-sm3@test.com', role: 'user' })
+
+      let members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const member = members.find(m => m.email === 'member-sm3@test.com')
+      assert.equal(member.department, 'dep1')
+
+      // re-invite without department — should move to org root
+      await ax.post('/api/invitations', { id: org.id, name: org.name, email: 'member-sm3@test.com', role: 'user' })
+
+      members = (await ax.get(`/api/organizations/${org.id}/members`)).data.results
+      const memberEntries = members.filter(m => m.email === 'member-sm3@test.com')
+      assert.equal(memberEntries.length, 1)
+      assert.equal(memberEntries[0].department, undefined)
+      assert.equal(memberEntries[0].departmentName, undefined)
+    } finally {
+      config.singleMembership = false
+      config.alwaysAcceptInvitation = false
+    }
+  })
+})