config: outputs must be unique

config/config.go

@@ -586,7 +586,18 @@ func (c *Config) Validate() error {
 	}
 
 	// Check that all outputs are valid
+	{
+		found := make(map[string]struct{})
 		for _, o := range c.Outputs {
+			// Verify the output is new
+			if _, ok := found[o.Name]; ok {
+				errs = append(errs, fmt.Errorf(
+					"%s: duplicate output. output names must be unique.",
+					o.Name))
+				continue
+			}
+			found[o.Name] = struct{}{}
+
 			var invalidKeys []string
 			valueKeyFound := false
 			for k := range o.RawConfig.Raw {
@@ -626,6 +637,7 @@ func (c *Config) Validate() error {
 				}
 			}
 		}
+	}
 
 	// Check that all variables are in the proper context
 	for source, rc := range c.rawConfigs() {

config/config_test.go

@@ -300,6 +300,13 @@ func TestConfigValidate_outputBadField(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_outputDuplicate(t *testing.T) {
+	c := testConfig(t, "validate-output-dup")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_pathVar(t *testing.T) {
 	c := testConfig(t, "validate-path-var")
 	if err := c.Validate(); err != nil {

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

@@ -0,0 +1,10 @@
+resource "aws_instance" "web" {
+}
+
+output "ip" {
+  value = "foo"
+}
+
+output "ip" {
+  value = "bar"
+}