[INTERNAL] ProjectGraph#addProject: Never ignore duplicates

As of today, we don't known of any use case for this

lib/graph/ProjectGraph.js

@@ -52,15 +52,11 @@ class ProjectGraph {
 	 *
 	 * @public
 	 * @param {@ui5/project/specifications/Project} project Project which should be added to the graph
-	 * @param {boolean} [ignoreDuplicates=false] Whether an error should be thrown when a duplicate project is added
 	 */
-	addProject(project, ignoreDuplicates) {
+	addProject(project) {
 		this._checkSealed();
 		const projectName = project.getName();
 		if (this._projects[projectName]) {
-			if (ignoreDuplicates) {
-				return;
-			}
 			throw new Error(
 				`Failed to add project ${projectName} to graph: A project with that name has already been added`);
 		}

test/lib/graph/ProjectGraph.js

@@ -161,23 +161,6 @@ test("addProject: Add duplicate", async (t) => {
 	t.is(res, project1, "Should return correct project");
 });
 
-test("addProject: Add duplicate with ignoreDuplicates", async (t) => {
-	const {ProjectGraph} = t.context;
-	const graph = new ProjectGraph({
-		rootProjectName: "my root project"
-	});
-	const project1 = await createProject("application.a");
-	graph.addProject(project1);
-
-	const project2 = await createProject("application.a");
-	t.notThrows(() => {
-		graph.addProject(project2, true);
-	}, "Should not throw when adding duplicates");
-
-	const res = graph.getProject("application.a");
-	t.is(res, project1, "Should return correct project");
-});
-
 test("addProject: Add project with integer-like name", async (t) => {
 	const {ProjectGraph} = t.context;
 	const graph = new ProjectGraph({