don't add duplicate proxy provider nodes

Fix the logic to add proxy provider nodes for implicitly passed in
providers. The missing continue allowed multiple nodes satisfying the
same provider address to be added to the graph. When attaching the
providers to resources, the fist one encountered would be used, which
could change each time the graph was built.

terraform/transform_provider.go

@@ -227,7 +227,7 @@ func (t *ProviderTransformer) Transform(g *Graph) error {
 				break
 			}
 
-			// see if this in  an inherited provider
+			// see if this is a proxy provider pointing to another concrete config
 			if p, ok := target.(*graphNodeProxyProvider); ok {
 				g.Remove(p)
 				target = p.Target()
@@ -708,10 +708,13 @@ func (t *ProviderConfigTransformer) addProxyProviders(g *Graph, c *configs.Confi
 
 		concreteProvider := t.providers[fullName]
 
-		// replace the concrete node with the provider passed in
-		if concreteProvider != nil && t.proxiable[fullName] {
-			g.Replace(concreteProvider, proxy)
-			t.providers[fullName] = proxy
+		// replace the concrete node with the provider passed in only if it is
+		// proxyable
+		if concreteProvider != nil {
+			if t.proxiable[fullName] {
+				g.Replace(concreteProvider, proxy)
+				t.providers[fullName] = proxy
+			}
 			continue
 		}