您可以将错误捕获为：

try
{  
   // below sql query
   $contact = Contact::find($id);
   // ...
   $contact->save();
   return redirect('contactus.php')->with('status', 'We received your message. We will get back to you soon.');
// if catch any sql error then below code will execute
} catch(\Illuminate\Database\QueryException $e) {
     // return $e->getMessage(); // error message
     return back()->withErrors(["post not successful, duplicate entry"]);
}

你可以用 try catch 查询异常：

try {
    $contact->save();
    return redirect('contactus.php')->with('status', 'We received your message. We will get back to you soon.');

} catch(\Illuminate\Database\QueryException $e){
    $errorCode = $e->errorInfo[1];
    if($errorCode == '1062'){
       return back()->withErrors(['Your message may be a duplicate. Did you refresh the page? We blocked that submission. If you feel this was in error, e-mail us or call us.']);
    }
    else{
     return back()->withErrors($e->getMessage());
    }
}

或者你可以先找到/检查数据，如果已经存在就发送错误。例子：

$contact = Contact::where('email',$request->email)->first();
if($contact)
{
   return back()->withErrors(['Your message may be a duplicate. Did you refresh the page? We blocked that submission. If you feel this was in error, e-mail us or call us.']);
}