fix(cli): surface clear error for duplicate -o instead of internal error

src/index.ts

@@ -751,7 +751,6 @@ function parseOptions(
             { argv, partial },
         );
     } catch (e) {
-        assert(!partial, "Partial option parsing should not have failed");
         return messageError("DriverCLIOptionParsingFailed", {
             message: exceptionToString(e),
         });

test/unit/cli-options.test.ts

@@ -0,0 +1,15 @@
+import { describe, expect, test } from "vitest";
+
+import { parseCLIOptions } from "../../src/index.js";
+
+describe("CLI option parsing", () => {
+    test("reports a duplicate output option as a user input error", () => {
+        expect(() =>
+            parseCLIOptions(["-o", "list.go", "-o", "list.ts"]),
+        ).toThrow(/^Option parsing failed: .*Singular option already set/);
+
+        expect(() =>
+            parseCLIOptions(["-o", "list.go", "-o", "list.ts"]),
+        ).not.toThrow("Internal error");
+    });
+});