diff --git a/caddy/serveridx_test.go b/caddy/serveridx_test.go
new file mode 100644
index 0000000000..f5271acfb7
--- /dev/null
+++ b/caddy/serveridx_test.go
@@ -0,0 +1,66 @@
+package caddy
+
+import (
+	"testing"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAssignServerIdxIncrements(t *testing.T) {
+	h := httpcaddyfile.Helper{State: map[string]any{}}
+	m1 := &FrankenPHPModule{}
+	m2 := &FrankenPHPModule{}
+
+	m1.assignServerIdx(h)
+	m2.assignServerIdx(h)
+
+	require.Equal(t, 1, m1.ServerIdx)
+	require.Equal(t, 2, m2.ServerIdx)
+}
+
+func TestRegisterModulesWithSameServerIdxShareOneServer(t *testing.T) {
+	app := &FrankenPHPApp{}
+	shared1 := &FrankenPHPModule{ServerIdx: 1, resolvedDocumentRoot: "../testdata"}
+	shared2 := &FrankenPHPModule{ServerIdx: 1, resolvedDocumentRoot: "../testdata"}
+	app.modules = []*FrankenPHPModule{shared1, shared2}
+
+	require.NoError(t, app.registerModules(caddy.NewReplacer()))
+
+	require.NotNil(t, shared1.server)
+	require.Same(t, shared1.server, shared2.server, "modules with the same server_idx must share one server instance")
+}
+
+func TestRegisterModulesWithoutServerIdxGetOwnServers(t *testing.T) {
+	app := &FrankenPHPApp{}
+	auto1 := &FrankenPHPModule{resolvedDocumentRoot: "../testdata"}
+	auto2 := &FrankenPHPModule{resolvedDocumentRoot: "../testdata"}
+	indexed := &FrankenPHPModule{ServerIdx: 1, resolvedDocumentRoot: "../testdata"}
+	app.modules = []*FrankenPHPModule{auto1, indexed, auto2}
+
+	require.NoError(t, app.registerModules(caddy.NewReplacer()))
+
+	require.NotNil(t, auto1.server)
+	require.NotNil(t, auto2.server)
+	require.NotNil(t, indexed.server)
+	require.NotSame(t, auto1.server, auto2.server, "modules without server_idx must each get their own server")
+	require.NotSame(t, auto1.server, indexed.server)
+	require.NotSame(t, auto2.server, indexed.server)
+}
+
+// regression test for the double-registration scenario: a module POSTed via
+// the admin API with an explicit server_idx must not steal the server of a
+// module that already registered under the same index; it joins it instead,
+// and the first registered module defines the server configuration
+func TestRegisterModulesFirstModuleWinsPerIdx(t *testing.T) {
+	app := &FrankenPHPApp{}
+	first := &FrankenPHPModule{ServerIdx: 2, resolvedDocumentRoot: "../testdata"}
+	second := &FrankenPHPModule{ServerIdx: 2, resolvedDocumentRoot: "../testdata/env"}
+	app.modules = []*FrankenPHPModule{first, second}
+
+	require.NoError(t, app.registerModules(caddy.NewReplacer()))
+
+	require.Same(t, first.server, second.server)
+	require.Len(t, app.opts, 1, "only one server must be registered for a shared index")
+}