fix: disable double pulls.

src/main.rs

@@ -1,7 +1,10 @@
 use crate::fetch_problem::{Fetcher, ProblemManager};
 use std::fs;
+use std::fs::exists;
 use std::io::Write;
 use std::path::Path;
+use anyhow::anyhow;
+use serde::de::Error;
 
 mod fetch_problem;
 
@@ -41,8 +44,13 @@ async fn main() {
     }
 }
 
-fn write_file(file_name: &String, file_content: &String) -> std::io::Result<()> {
+fn write_file(file_name: &String, file_content: &String) -> anyhow::Result<()> {
     let file_path = Path::new("./src/problem").join(format!("{}.rs", file_name));
+    
+    if exists(&file_path)? {
+        println!("{} has pulled.", file_name);
+        return Err(anyhow!("{} has pulled", file_name));
+    }
 
     let mut file = fs::OpenOptions::new()
         .write(true)
@@ -58,5 +66,6 @@ fn write_file(file_name: &String, file_content: &String) -> std::io::Result<()>
         .append(true)
         .open("./src/problem/mod.rs")?;
 
-    write!(mod_file, "\nmod {};", file_name)
+    write!(mod_file, "\nmod {};", file_name)?;
+    Ok(())
 }