Do not copy a file if it already exists.

src/context/dependency.rs

@@ -27,8 +27,17 @@ impl Dependency {
                         .ok_or("Output file should have a containing directory.")?,
                 )
                 .await?;
-                // TODO: If file already exists, either error out or compare hash to avoid duplicate write.
+
-                tokio::fs::copy(absolute_path, path_to_output).await?;
+                if tokio::fs::metadata(&path_to_output).await.is_ok() {
+                    // TODO: compare hash and error out if they do not match.
+                    println!(
+                        "Not copying {} to {} because the output file already exists.",
+                        absolute_path.display(),
+                        path_to_output.display()
+                    );
+                } else {
+                    tokio::fs::copy(absolute_path, path_to_output).await?;
+                }
                 Ok(())
             }
         }