config: resource should be unique

config/config.go

@@ -145,7 +145,18 @@ func (c *Config) Validate() error {
 
 	// Check that all references to resources are valid
 	resources := make(map[string]*Resource)
+	dupped := make(map[string]struct{})
 	for _, r := range c.Resources {
+		if _, ok := resources[r.Id()]; ok {
+			if _, ok := dupped[r.Id()]; !ok {
+				dupped[r.Id()] = struct{}{}
+
+				errs = append(errs, fmt.Errorf(
+					"%s: resource repeated multiple times",
+					r.Id()))
+			}
+		}
+
 		resources[r.Id()] = r
 	}
 	for source, vs := range vars {

config/config_test.go

@@ -22,6 +22,13 @@ func TestConfigValidate_badMultiResource(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_dupResource(t *testing.T) {
+	c := testConfig(t, "validate-dup-resource")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_outputBadField(t *testing.T) {
 	c := testConfig(t, "validate-output-bad-field")
 	if err := c.Validate(); err == nil {

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

@@ -0,0 +1,7 @@
+resource "aws_instance" "web" {
+  count = 5
+}
+
+resource "aws_instance" "web" {
+  count = 10
+}