config: validate that a multi provider is only configured once each

config/config.go

@@ -239,6 +239,20 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	// Check that providers aren't declared multiple times.
+	providerSet := make(map[string]struct{})
+	for _, p := range c.ProviderConfigs {
+		name := p.FullName()
+		if _, ok := providerSet[name]; ok {
+			errs = append(errs, fmt.Errorf(
+				"provider.%s: declared multiple times, you can only declare a provider once",
+				name))
+			continue
+		}
+
+		providerSet[name] = struct{}{}
+	}
+
 	// Check that all references to modules are valid
 	modules := make(map[string]*Module)
 	dupped := make(map[string]struct{})
@@ -651,6 +665,14 @@ func (o *Output) mergerMerge(m merger) merger {
 	return &result
 }
 
+func (c *ProviderConfig) FullName() string {
+	if c.Alias == "" {
+		return c.Name
+	}
+
+	return fmt.Sprintf("%s.%s", c.Name, c.Alias)
+}
+
 func (c *ProviderConfig) mergerName() string {
 	return c.Name
 }

config/config_test.go

@@ -203,6 +203,13 @@ func TestConfigValidate_pathVarInvalid(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_providerMulti(t *testing.T) {
+	c := testConfig(t, "validate-provider-multi")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_provConnSplatOther(t *testing.T) {
 	c := testConfig(t, "validate-prov-conn-splat-other")
 	if err := c.Validate(); err != nil {

config/test-fixtures/validate-provider-multi/main.tf

@@ -0,0 +1,7 @@
+provider "aws" {
+    alias = "foo"
+}
+
+provider "aws" {
+    alias = "foo"
+}