[FIX] ProjectGraph: Improve error message when adding duplicate proje…

…cts or extensions

lib/graph/ProjectGraph.js

@@ -58,7 +58,8 @@ class ProjectGraph {
 		const projectName = project.getName();
 		if (this._projects.has(projectName)) {
 			throw new Error(
-				`Failed to add project ${projectName} to graph: A project with that name has already been added`);
+				`Failed to add project ${projectName} to graph: A project with that name has already been added. ` +
+				`This might be caused by multiple modules containing projects with the same name`);
 		}
 		if (!isNaN(projectName)) {
 			// Reject integer-like project names. They would take precedence when traversing object keys which
@@ -126,7 +127,8 @@ class ProjectGraph {
 		if (this._extensions.has(extensionName)) {
 			throw new Error(
 				`Failed to add extension ${extensionName} to graph: ` +
-				`An extension with that name has already been added`);
+				`An extension with that name has already been added. ` +
+				`This might be caused by multiple modules containing extensions with the same name`);
 		}
 		if (!isNaN(extensionName)) {
 			// Reject integer-like extension names. They would take precedence when traversing object keys which

test/lib/graph/ProjectGraph.js

@@ -154,7 +154,8 @@ test("addProject: Add duplicate", async (t) => {
 		graph.addProject(project2);
 	});
 	t.is(error.message,
-		"Failed to add project application.a to graph: A project with that name has already been added",
+		"Failed to add project application.a to graph: A project with that name has already been added. " +
+		"This might be caused by multiple modules containing projects with the same name",
 		"Should throw with expected error message");
 
 	const res = graph.getProject("application.a");
@@ -261,7 +262,8 @@ test("addExtension: Add duplicate", async (t) => {
 		graph.addExtension(extension2);
 	});
 	t.is(error.message,
-		"Failed to add extension extension.a to graph: An extension with that name has already been added",
+		"Failed to add extension extension.a to graph: An extension with that name has already been added. " +
+		"This might be caused by multiple modules containing extensions with the same name",
 		"Should throw with expected error message");
 
 	const res = graph.getExtension("extension.a");

test/lib/graph/projectGraphBuilder.js

@@ -961,6 +961,8 @@ test("Multiple dependencies to different module containing the same extension",
 		}),
 	]);
 	await t.throwsAsync(projectGraphBuilder(t.context.provider), {
-		message: "Failed to add extension task-a to graph: An extension with that name has already been added"
+		message:
+			"Failed to add extension task-a to graph: An extension with that name has already been added. " +
+			"This might be caused by multiple modules containing extensions with the same name"
 	});
 });