skip already added provider nodes

We can skip providers which already have a node in the graph for their
type.

internal/terraform/transform_provider.go

@@ -543,6 +543,13 @@ func (t *ProviderConfigTransformer) transformSingle(g *Graph, c *configs.Config)
 				Module:   path,
 			}
 
+			if _, ok := t.providers[addr.String()]; ok {
+				// The config validation warns about this too, but we can't
+				// completely prevent it in v1.
+				log.Printf("[WARN] ProviderConfigTransformer: duplicate required_providers entry for %s", addr)
+				continue
+			}
+
 			abstract := &NodeAbstractProvider{
 				Addr: addr,
 			}
@@ -568,6 +575,13 @@ func (t *ProviderConfigTransformer) transformSingle(g *Graph, c *configs.Config)
 			Module:   path,
 		}
 
+		if _, ok := t.providers[addr.String()]; ok {
+			// The abstract provider node may already have been added from the
+			// provider requirements.
+			log.Printf("[WARN] ProviderConfigTransformer: provider node %s already added", addr)
+			continue
+		}
+
 		abstract := &NodeAbstractProvider{
 			Addr: addr,
 		}

internal/terraform/transform_provider_test.go

@@ -446,6 +446,42 @@ provider["registry.terraform.io/hashicorp/test"].z`
 	}
 }
 
+func TestProviderConfigTransformer_duplicateLocalName(t *testing.T) {
+	mod := testModuleInline(t, map[string]string{
+		"main.tf": `
+terraform {
+  required_providers {
+	# We have to allow this since it wasn't previously prevented. If the
+	# default config is equivalent to the provider config, the user may never
+	# see an error.
+    dupe = {
+      source = "registry.terraform.io/hashicorp/test"
+    }
+  }
+}
+
+provider "test" {
+}
+`})
+	concrete := func(a *NodeAbstractProvider) dag.Vertex { return a }
+
+	g := testProviderTransformerGraph(t, mod)
+	tf := ProviderConfigTransformer{
+		Config:   mod,
+		Concrete: concrete,
+	}
+	if err := tf.Transform(g); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	expected := `provider["registry.terraform.io/hashicorp/test"]`
+
+	actual := strings.TrimSpace(g.String())
+	if actual != expected {
+		t.Fatalf("expected:\n%s\n\ngot:\n%s", expected, actual)
+	}
+}
+
 const testTransformProviderBasicStr = `
 aws_instance.web
   provider["registry.terraform.io/hashicorp/aws"]