diff --git a/docs/release-control/v6/internal/IDENTITY_INVARIANTS.md b/docs/release-control/v6/internal/IDENTITY_INVARIANTS.md
index 7b683db86..0eadb98a1 100644
--- a/docs/release-control/v6/internal/IDENTITY_INVARIANTS.md
+++ b/docs/release-control/v6/internal/IDENTITY_INVARIANTS.md
@@ -83,7 +83,9 @@ principal once a stable user ID exists.
    contact email at verification time, not to the email embedded in the token.
    If the matching organization owner/member has no stored principal, the
    magic-link flow must fail closed instead of synthesizing a principal from
-   contact email.
+   contact email. If one contact email maps to multiple distinct stored
+   principals, email-based resolution must also fail closed instead of choosing
+   a principal by row order.
 3. Hosted checkout and tenant provisioning seed organization membership from
    stable Pulse user IDs. Registry-backed paths must create or resolve the
    registry user before writing hosted tenant `OwnerUserID` or member `UserID`;
diff --git a/docs/release-control/v6/internal/subsystems/organization-settings.md b/docs/release-control/v6/internal/subsystems/organization-settings.md
index acb8d64fb..8c9c16938 100644
--- a/docs/release-control/v6/internal/subsystems/organization-settings.md
+++ b/docs/release-control/v6/internal/subsystems/organization-settings.md
@@ -184,6 +184,9 @@ helper so handlers do not duplicate membership lookup or accidentally bind
 sessions to email. A matching owner/member contact email without a stored
 `OwnerUserID` or member `UserID` is not a principal; the helper must fail
 closed instead of manufacturing a session key from email.
+If one contact email matches multiple distinct stored principals, the model
+must also fail closed instead of selecting a principal by owner/member row
+order.
 Organization-principal canonicalization may use email only when the stored
 owner/member principal is itself legacy email-keyed. A contact email on an
 already-stable owner/member must not authorize, replace, or migrate a different
diff --git a/internal/api/identity_invariant_contract_test.go b/internal/api/identity_invariant_contract_test.go
index 590badcc6..83cf664c9 100644
--- a/internal/api/identity_invariant_contract_test.go
+++ b/internal/api/identity_invariant_contract_test.go
@@ -139,6 +139,7 @@ func TestContract_HostedIdentityUsesStablePrincipals(t *testing.T) {
 			"CanonicalizePrincipalIdentity",
 			"ownerMatchesLegacyEmailPrincipal",
 			"memberMatchesLegacyEmailPrincipal",
+			"mergePrincipalEmailCandidate",
 			"return \"\", \"\", false",
 		},
 		"../../docs/release-control/v6/internal/IDENTITY_INVARIANTS.md": {
@@ -147,6 +148,7 @@ func TestContract_HostedIdentityUsesStablePrincipals(t *testing.T) {
 			"SSO provider subject",
 			"reject caller-supplied metadata",
 			"contact email attached to an already-stable owner/member principal must not",
+			"maps to multiple distinct stored",
 			"Self-hosted SSO sessions now use provider-scoped",
 		},
 	}
diff --git a/internal/models/organization.go b/internal/models/organization.go
index f1a677f51..e0811f941 100644
--- a/internal/models/organization.go
+++ b/internal/models/organization.go
@@ -330,6 +330,26 @@ func memberMatchesLegacyEmailPrincipal(member OrganizationMember, email string)
 	return normalizeOrganizationEmail(userID) == normalizeOrganizationEmail(email)
 }
 
+func mergePrincipalEmailCandidate(resolvedUserID *string, resolvedRole *OrganizationRole, userID string, role OrganizationRole) bool {
+	userID = normalizeOrganizationIdentityValue(userID)
+	if userID == "" {
+		return false
+	}
+	role = NormalizeOrganizationRole(role)
+	if *resolvedUserID == "" {
+		*resolvedUserID = userID
+		*resolvedRole = role
+		return true
+	}
+	if *resolvedUserID != userID {
+		return false
+	}
+	if OrganizationRoleAtLeast(role, *resolvedRole) {
+		*resolvedRole = role
+	}
+	return true
+}
+
 // HasMember checks if a user is a member of the organization.
 func (o *Organization) HasMember(userID string) bool {
 	for _, member := range o.Members {
@@ -443,24 +463,25 @@ func (o *Organization) ResolvePrincipalByEmail(email string) (string, Organizati
 	if email == "" {
 		return "", "", false
 	}
+	userID := ""
+	role := OrganizationRole("")
 	if ownerMatchesEmail(o, email) {
-		userID := normalizeOrganizationIdentityValue(o.OwnerUserID)
-		if userID == "" {
+		if !mergePrincipalEmailCandidate(&userID, &role, o.OwnerUserID, OrgRoleOwner) {
 			return "", "", false
 		}
-		return userID, OrgRoleOwner, true
 	}
 	for _, member := range o.Members {
 		if !memberMatchesEmail(member, email) {
 			continue
 		}
-		userID := normalizeOrganizationIdentityValue(member.UserID)
-		if userID == "" {
+		if !mergePrincipalEmailCandidate(&userID, &role, member.UserID, member.Role) {
 			return "", "", false
 		}
-		return userID, NormalizeOrganizationRole(member.Role), true
 	}
-	return "", "", false
+	if userID == "" {
+		return "", "", false
+	}
+	return userID, role, true
 }
 
 // CanonicalizePrincipalIdentity upgrades legacy email-keyed membership records
diff --git a/internal/models/organization_additional_test.go b/internal/models/organization_additional_test.go
index fe02374fb..b5181adce 100644
--- a/internal/models/organization_additional_test.go
+++ b/internal/models/organization_additional_test.go
@@ -169,6 +169,34 @@ func TestOrganizationResolvePrincipalByEmail(t *testing.T) {
 	}
 }
 
+func TestOrganizationResolvePrincipalByEmailRejectsAmbiguousStoredPrincipals(t *testing.T) {
+	ownerCollision := &Organization{
+		ID:          "org-owner-collision",
+		OwnerUserID: "u_owner",
+		OwnerEmail:  "shared@example.com",
+		Members: []OrganizationMember{
+			{UserID: "u_owner", Email: "shared@example.com", Role: OrgRoleOwner},
+			{UserID: "u_admin", Email: "shared@example.com", Role: OrgRoleAdmin},
+		},
+	}
+	userID, role, ok := ownerCollision.ResolvePrincipalByEmail("shared@example.com")
+	if ok || userID != "" || role != "" {
+		t.Fatalf("owner collision principal = (%q, %q, %v), want rejection", userID, role, ok)
+	}
+
+	memberCollision := &Organization{
+		ID: "org-member-collision",
+		Members: []OrganizationMember{
+			{UserID: "u_admin", Email: "shared@example.com", Role: OrgRoleAdmin},
+			{UserID: "u_viewer", Email: "shared@example.com", Role: OrgRoleViewer},
+		},
+	}
+	userID, role, ok = memberCollision.ResolvePrincipalByEmail("shared@example.com")
+	if ok || userID != "" || role != "" {
+		t.Fatalf("member collision principal = (%q, %q, %v), want rejection", userID, role, ok)
+	}
+}
+
 func TestOrganizationResolvePrincipalByEmailRejectsBlankStoredPrincipal(t *testing.T) {
 	ownerOnlyEmail := &Organization{
 		ID:         "org-blank-owner",