config: validate no duplicate modules

config/config.go

@@ -173,9 +173,27 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	// Check that all references to modules are valid
+	modules := make(map[string]*Module)
+	dupped := make(map[string]struct{})
+	for _, m := range c.Modules {
+		if _, ok := modules[m.Id()]; ok {
+			if _, ok := dupped[m.Id()]; !ok {
+				dupped[m.Id()] = struct{}{}
+
+				errs = append(errs, fmt.Errorf(
+					"%s: module repeated multiple times",
+					m.Id()))
+			}
+		}
+
+		modules[m.Id()] = m
+	}
+	dupped = nil
+
 	// Check that all references to resources are valid
 	resources := make(map[string]*Resource)
-	dupped := make(map[string]struct{})
+	dupped = make(map[string]struct{})
 	for _, r := range c.Resources {
 		if _, ok := resources[r.Id()]; ok {
 			if _, ok := dupped[r.Id()]; !ok {

config/config_test.go

@@ -44,6 +44,13 @@ func TestConfigValidate_countZero(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_dupModule(t *testing.T) {
+	c := testConfig(t, "validate-dup-module")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_dupResource(t *testing.T) {
 	c := testConfig(t, "validate-dup-resource")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-dup-module/main.tf

@@ -0,0 +1,7 @@
+module "aws_instance" "web" {
+  source = "foo"
+}
+
+module "aws_instance" "web" {
+  source = "bar"
+}