diff --git a/compiler/ctx.ts b/compiler/ctx.ts
index 5d0267b..001378c 100644
--- a/compiler/ctx.ts
+++ b/compiler/ctx.ts
@@ -1,11 +1,11 @@
 import * as ast from "./ast/mod.ts";
-import { Diag } from "./diagnostics.ts";
+import { prettyPrintReport, printStackTrace, Report } from "./diagnostics.ts";
 
 export class Ctx {
     private fileIds = new Ids();
     private files = new Map<Id<File>, FileInfo>();
 
-    private diags: Diag[] = [];
+    private reports: Report[] = [];
 
     public fileHasChildWithIdent(file: File, childIdent: string): boolean {
         return this.files.get(id(file))!
@@ -40,43 +40,23 @@ export class Ctx {
     }
 
     public fileInfo(file: File): FileInfo {
-        this.files.get(id(file))!;
+        return this.files.get(id(file))!;
     }
 
-    public reportFatal(file: File, msg: string) {
-        console.error(`fatal: ${msg}`);
-        this.reportImmediately(file, msg);
+    public report(rep: Report) {
+        this.reports.push(rep);
+        this.reportImmediately(rep);
     }
 
     public enableReportImmediately = false;
     public enableStacktrace = false;
-    private reportImmediately(file: File, msg: string) {
-        if (!this.enableReportImmediately) {
-            return;
-        }
-
-        if (!this.enableStacktrace) {
-            return;
-        }
-        class StackTracer extends Error {
-            constructor() {
-                super("StackTracer");
+    private reportImmediately(rep: Report) {
+        if (this.enableReportImmediately) {
+            prettyPrintReport(this, rep);
+            if (this.enableStacktrace) {
+                printStackTrace();
             }
         }
-        try {
-            //throw new ReportNotAnError();
-        } catch (error) {
-            if (!(error instanceof StackTracer)) {
-                throw error;
-            }
-            console.log(
-                error.stack?.replace(
-                    "Error: StackTracer",
-                    "Stack trace:",
-                ) ??
-                    error,
-            );
-        }
     }
 }
 
diff --git a/compiler/diagnostics.ts b/compiler/diagnostics.ts
index 080c030..6170397 100644
--- a/compiler/diagnostics.ts
+++ b/compiler/diagnostics.ts
@@ -11,7 +11,7 @@ export type Pos = {
     col: number;
 };
 
-export type Diag = {
+export type Report = {
     severity: "fatal" | "error" | "warning" | "info";
     origin?: string;
     msg: string;
@@ -20,24 +20,35 @@ export type Diag = {
     pos?: Pos;
 };
 
-export function prettyPrintDiag(ctx: Ctx, diag: Diag) {
-    const { severity, msg } = diag;
-    const origin = diag.origin ? `${diag.origin}: ` : "";
+export function prettyPrintReport(ctx: Ctx, rep: Report) {
+    const { severity, msg } = rep;
+    const origin = rep.origin ? `${rep.origin}: ` : "";
     console.error(`${origin}${severity}: ${msg}`);
-    if (diag.file && (diag.span || diag.pos)) {
-        const { absPath: path, text } = ctx.fileInfo(diag.file);
-        const { line, col } = diag.span?.begin ?? diag.pos!;
+    if (rep.file && (rep.span || rep.pos)) {
+        const { absPath: path } = ctx.fileInfo(rep.file);
+        const { line, col } = rep.span?.begin ?? rep.pos!;
         console.error(`    at ./${path}:${line}:${col}`);
-        if (diag.span) {
-            let begin = diag.span.begin.idx;
-            while (begin >= 0 && text[begin - 1] != "\n") {
-                begin -= 1;
-            }
-            let end = diag.span.end.idx;
-            while (end < text.length && text[end + 1] != "\n") {
-                end += 1;
-            }
-        } else if (diag.pos) {
-        }
+    }
+}
+
+export function printStackTrace() {
+    class StackTracer extends Error {
+        constructor() {
+            super("StackTracer");
+        }
+    }
+    try {
+        throw new StackTracer();
+    } catch (error) {
+        if (!(error instanceof StackTracer)) {
+            throw error;
+        }
+        console.log(
+            error.stack?.replace(
+                "Error: StackTracer",
+                "Stack trace:",
+            ) ??
+                error,
+        );
     }
 }
diff --git a/compiler/main.ts b/compiler/main.ts
index c0b7153..fb99755 100644
--- a/compiler/main.ts
+++ b/compiler/main.ts
@@ -61,6 +61,11 @@ export class FileTreeCollector implements ast.Visitor<[_P]> {
         this.subFilePromise = this.subFilePromise
             .then(() => {
                 if (this.ctx.fileHasChildWithIdent(file, ident)) {
+                    this.ctx.report({
+                        severity: "fatal",
+                        msg: `module '${ident}' already declared`,
+                        file,
+                    });
                 }
                 return new FileTreeCollector(
                     this.ctx,