Fix duplicated PATH entries

Author: chansuke

src/env_var.rs

@@ -27,6 +27,8 @@ pub fn prepend_path(name: &str, value: Vec<PathBuf>, cmd: &mut Command) {
     if let Some(ref v) = old_value {
         parts = value;
         parts.extend(env::split_paths(v).collect::<Vec<_>>());
+        parts.sort();
+        parts.dedup();
     } else {
         parts = value;
     }