diff --git a/src/env_var.rs b/src/env_var.rs
index ab48f5bc88..ccf1b5ca9f 100644
--- a/src/env_var.rs
+++ b/src/env_var.rs
@@ -1,5 +1,6 @@
-use std::collections::VecDeque;
+use std::collections::{HashSet, VecDeque};
 use std::env;
+use std::ffi::OsStr;
 use std::path::PathBuf;
 use std::process::Command;
 
@@ -7,13 +8,34 @@ use crate::process;
 
 pub const RUST_RECURSION_COUNT_MAX: u32 = 20;
 
+/// This can be removed when https://github.com/rust-lang/rust/issues/44434 is
+/// stablised.
+pub(crate) trait ProcessEnvs {
+    fn env<K, V>(&mut self, key: K, val: V) -> &mut Self
+    where
+        Self: Sized,
+        K: AsRef<OsStr>,
+        V: AsRef<OsStr>;
+}
+
+impl ProcessEnvs for Command {
+    fn env<K, V>(&mut self, key: K, val: V) -> &mut Self
+    where
+        Self: Sized,
+        K: AsRef<OsStr>,
+        V: AsRef<OsStr>,
+    {
+        self.env(key, val)
+    }
+}
+
 #[allow(unused)]
-fn append_path(name: &str, value: Vec<PathBuf>, cmd: &mut Command) {
+fn append_path<E: ProcessEnvs>(name: &str, value: Vec<PathBuf>, cmd: &mut E) {
     let old_value = process().var_os(name);
     let mut parts: Vec<PathBuf>;
     if let Some(ref v) = old_value {
-        parts = env::split_paths(v).collect();
-        parts.extend(value);
+        let old_paths: Vec<PathBuf> = env::split_paths(v).collect::<Vec<_>>();
+        parts = concat_uniq_paths(old_paths, value);
     } else {
         parts = value;
     }
@@ -22,7 +44,7 @@ fn append_path(name: &str, value: Vec<PathBuf>, cmd: &mut Command) {
     }
 }
 
-pub(crate) fn prepend_path(name: &str, prepend: Vec<PathBuf>, cmd: &mut Command) {
+pub(crate) fn prepend_path<E: ProcessEnvs>(name: &str, prepend: Vec<PathBuf>, cmd: &mut E) {
     let old_value = process().var_os(name);
     let parts = if let Some(ref v) = old_value {
         let mut tail = env::split_paths(v).collect::<VecDeque<_>>();
@@ -50,3 +72,167 @@ pub(crate) fn inc(name: &str, cmd: &mut Command) {
 
     cmd.env(name, (old_value + 1).to_string());
 }
+
+fn concat_uniq_paths(fst_paths: Vec<PathBuf>, snd_paths: Vec<PathBuf>) -> Vec<PathBuf> {
+    let deduped_fst_paths = dedupe_with_preserved_order(fst_paths);
+    let deduped_snd_paths = dedupe_with_preserved_order(snd_paths);
+
+    let vec_fst_paths: Vec<_> = deduped_fst_paths.into_iter().collect();
+
+    let mut unified_paths;
+    unified_paths = vec_fst_paths.clone();
+    unified_paths.extend(
+        deduped_snd_paths
+            .into_iter()
+            .filter(|v| !vec_fst_paths.contains(v))
+            .collect::<Vec<_>>(),
+    );
+
+    unified_paths
+}
+
+fn dedupe_with_preserved_order(paths: Vec<PathBuf>) -> Vec<PathBuf> {
+    let mut uniq_paths: Vec<PathBuf> = Vec::new();
+    let mut seen_paths: HashSet<PathBuf> = HashSet::new();
+
+    for path in &paths {
+        if !seen_paths.contains(path) {
+            seen_paths.insert(path.to_path_buf());
+            uniq_paths.push(path.to_path_buf());
+        }
+    }
+
+    uniq_paths
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::currentprocess;
+    use crate::test::{with_saved_path, Env};
+
+    use super::ProcessEnvs;
+    use std::collections::HashMap;
+    use std::ffi::{OsStr, OsString};
+
+    #[derive(Default)]
+    struct TestCommand {
+        envs: HashMap<OsString, Option<OsString>>,
+    }
+
+    impl ProcessEnvs for TestCommand {
+        fn env<K, V>(&mut self, key: K, val: V) -> &mut Self
+        where
+            Self: Sized,
+            K: AsRef<OsStr>,
+            V: AsRef<OsStr>,
+        {
+            self.envs
+                .insert(key.as_ref().to_owned(), Some(val.as_ref().to_owned()));
+            self
+        }
+    }
+
+    #[test]
+    fn deduplicate_and_concat_paths() {
+        let mut old_paths = vec![];
+
+        let z = OsString::from("/home/z/.cargo/bin");
+        let path_z = PathBuf::from(z);
+        old_paths.push(path_z);
+
+        let a = OsString::from("/home/a/.cargo/bin");
+        let path_a = PathBuf::from(a);
+        old_paths.push(path_a);
+
+        let _a = OsString::from("/home/a/.cargo/bin");
+        let _path_a = PathBuf::from(_a);
+        old_paths.push(_path_a);
+
+        let mut new_paths = vec![];
+
+        let n = OsString::from("/home/n/.cargo/bin");
+        let path_n = PathBuf::from(n);
+        new_paths.push(path_n);
+
+        let g = OsString::from("/home/g/.cargo/bin");
+        let path_g = PathBuf::from(g);
+        new_paths.push(path_g);
+
+        let _g = OsString::from("/home/g/.cargo/bin");
+        let _path_g = PathBuf::from(_g);
+        new_paths.push(_path_g);
+
+        let _z = OsString::from("/home/z/.cargo/bin");
+        let path_z = PathBuf::from(_z);
+        old_paths.push(path_z);
+
+        let mut unified_paths: Vec<PathBuf> = vec![];
+        let zpath = OsString::from("/home/z/.cargo/bin");
+        let _zpath = PathBuf::from(zpath);
+        unified_paths.push(_zpath);
+        let apath = OsString::from("/home/a/.cargo/bin");
+        let _apath = PathBuf::from(apath);
+        unified_paths.push(_apath);
+        let npath = OsString::from("/home/n/.cargo/bin");
+        let _npath = PathBuf::from(npath);
+        unified_paths.push(_npath);
+        let gpath = OsString::from("/home/g/.cargo/bin");
+        let _gpath = PathBuf::from(gpath);
+        unified_paths.push(_gpath);
+
+        assert_eq!(concat_uniq_paths(old_paths, new_paths), unified_paths);
+    }
+
+    #[test]
+    fn prepend_unique_path() {
+        let mut vars = HashMap::new();
+        vars.env(
+            "PATH",
+            env::join_paths(vec!["/home/a/.cargo/bin", "/home/b/.cargo/bin"].iter()).unwrap(),
+        );
+        let tp = Box::new(currentprocess::TestProcess {
+            vars,
+            ..Default::default()
+        });
+        with_saved_path(&|| {
+            currentprocess::with(tp.clone(), || {
+                let mut path_entries = vec![];
+                let mut cmd = TestCommand::default();
+
+                let a = OsString::from("/home/a/.cargo/bin");
+                let path_a = PathBuf::from(a);
+                path_entries.push(path_a);
+
+                let _a = OsString::from("/home/a/.cargo/bin");
+                let _path_a = PathBuf::from(_a);
+                path_entries.push(_path_a);
+
+                let z = OsString::from("/home/z/.cargo/bin");
+                let path_z = PathBuf::from(z);
+                path_entries.push(path_z);
+
+                prepend_path("PATH", path_entries, &mut cmd);
+                let envs: Vec<_> = cmd.envs.iter().collect();
+
+                assert_eq!(
+                    envs,
+                    &[(
+                        &OsString::from("PATH"),
+                        &Some(
+                            env::join_paths(
+                                vec![
+                                    "/home/z/.cargo/bin",
+                                    "/home/a/.cargo/bin",
+                                    "/home/b/.cargo/bin"
+                                ]
+                                .iter()
+                            )
+                            .unwrap()
+                        )
+                    ),]
+                );
+            });
+        });
+    }
+}