diff --git a/src/cargo/ops/cargo_new.rs b/src/cargo/ops/cargo_new.rs
index 7bef7e8a665..76473403be7 100644
--- a/src/cargo/ops/cargo_new.rs
+++ b/src/cargo/ops/cargo_new.rs
@@ -2,6 +2,7 @@ use std::collections::BTreeMap;
 use std::env;
 use std::fmt;
 use std::fs;
+use std::io::{BufReader, BufRead, ErrorKind};
 use std::path::{Path, PathBuf};
 
 use git2::Config as GitConfig;
@@ -409,69 +410,117 @@ pub fn init(opts: &NewOptions, config: &Config) -> CargoResult<()> {
     Ok(())
 }
 
-fn mk(config: &Config, opts: &MkOptions<'_>) -> CargoResult<()> {
-    let path = opts.path;
-    let name = opts.name;
-    let cfg = global_config(config)?;
-    // Please ensure that ignore and hgignore are in sync.
-    let ignore = [
-        "/target\n",
-        "**/*.rs.bk\n",
-        if !opts.bin { "Cargo.lock\n" } else { "" },
-    ]
-    .concat();
-    // Mercurial glob ignores can't be rooted, so just sticking a 'syntax: glob' at the top of the
-    // file will exclude too much. Instead, use regexp-based ignores. See 'hg help ignore' for
-    // more.
-    let hgignore = [
-        "^target/\n",
-        "glob:*.rs.bk\n",
-        if !opts.bin { "glob:Cargo.lock\n" } else { "" },
-    ]
-    .concat();
 
-    let vcs = opts.version_control.unwrap_or_else(|| {
-        let in_existing_vcs = existing_vcs_repo(path.parent().unwrap_or(path), config.cwd());
-        match (cfg.version_control, in_existing_vcs) {
-            (None, false) => VersionControl::Git,
-            (Some(opt), false) => opt,
-            (_, true) => VersionControl::NoVcs,
+/// IgnoreList
+struct IgnoreList {
+    /// git like formatted entries
+    ignore: Vec<String>,
+    /// mercurial formatted entries
+    hg_ignore: Vec<String>,
+}
+
+impl IgnoreList {
+    /// constructor to build a new ignore file
+    fn new() -> IgnoreList {
+        return IgnoreList{
+            ignore: Vec::new(), 
+            hg_ignore: Vec::new(),
         }
-    });
+    }
+
+    /// add a new entry to the ignore list. Requires two arguments with the 
+    /// entry in two different formats. One for "git style" entries and one for
+    /// "mercurial like" entries.
+    fn push(&mut self, ignore: &str, hg_ignore: &str) {
+        self.ignore.push(ignore.to_string());
+        self.hg_ignore.push(hg_ignore.to_string());
+    }
+
+    /// Return the correctly formatted content of the ignore file for the given
+    /// version control system as `String`.
+    fn format_new(&self, vcs: VersionControl) -> String {
+        match vcs {
+            VersionControl::Hg => return self.hg_ignore.join("\n"),
+            _ => return self.ignore.join("\n"),
+        };
+    }
+
+    /// format_existing is used to format the IgnoreList when the ignore file
+    /// already exists. It reads the contents of the given `BufRead` and
+    /// checks if the contents of the ignore list are already existing in the
+    /// file.
+    fn format_existing<T: BufRead>(&self, existing: T, vcs: VersionControl) -> String {
+        // TODO: is unwrap safe?
+        let existing_items = existing.lines().collect::<Result<Vec<_>, _>>().unwrap(); 
+
+        let ignore_items = match vcs { 
+            VersionControl::Hg =>  &self.hg_ignore,
+            _ =>  &self.ignore,
+        };
+
+        let mut out = "\n\n#Added by cargo\n\
+                      #\n\
+                      #already existing elements are commented out\n".
+                      to_string();
+
+        for item in ignore_items {
+            out.push('\n');
+            if existing_items.contains(item) {
+                out.push('#');
+            } 
+            out.push_str(item)
+        }
+
+        out
+    }
+}
+
+/// write the ignore file to the given directory. If the ignore file for the
+/// given vcs system already exists, its content is read and duplicate ignore
+/// file entries are filtered out.
+fn write_ignore_file(base_path: &Path, list: &IgnoreList, vcs: VersionControl) -> CargoResult<String>{
+    let fp_ignore = match vcs {
+        VersionControl::Git => base_path.join(".gitignore"),
+        VersionControl::Hg =>  base_path.join(".hgignore"),
+        VersionControl::Pijul =>  base_path.join(".ignore"),
+        VersionControl::Fossil => return Ok("".to_string()),
+        VersionControl::NoVcs =>  return Ok("".to_string()),
+    };
+
+    let ignore: String = match fs::File::open(&fp_ignore) {
+        Err(why) => {
+            match why.kind() {
+                ErrorKind::NotFound => list.format_new(vcs),
+                _ => return Err(failure::format_err!("{}", why)),
+            }
+        },
+        Ok(file) => {
+            list.format_existing(BufReader::new(file), vcs)
+        },
+    };
 
+    paths::append(&fp_ignore, ignore.as_bytes())?;
+
+    return Ok(ignore)
+}
+
+/// initialize the correct vcs system based on the provided config
+fn init_vcs(path: &Path, vcs: VersionControl, config: &Config) -> CargoResult<()> {
     match vcs {
         VersionControl::Git => {
             if !path.join(".git").exists() {
                 GitRepo::init(path, config.cwd())?;
             }
-            let ignore = if path.join(".gitignore").exists() {
-                format!("\n{}", ignore)
-            } else {
-                ignore
-            };
-            paths::append(&path.join(".gitignore"), ignore.as_bytes())?;
         }
         VersionControl::Hg => {
             if !path.join(".hg").exists() {
                 HgRepo::init(path, config.cwd())?;
             }
-            let hgignore = if path.join(".hgignore").exists() {
-                format!("\n{}", hgignore)
-            } else {
-                hgignore
-            };
-            paths::append(&path.join(".hgignore"), hgignore.as_bytes())?;
         }
         VersionControl::Pijul => {
             if !path.join(".pijul").exists() {
                 PijulRepo::init(path, config.cwd())?;
             }
-            let ignore = if path.join(".ignore").exists() {
-                format!("\n{}", ignore)
-            } else {
-                ignore
-            };
-            paths::append(&path.join(".ignore"), ignore.as_bytes())?;
         }
         VersionControl::Fossil => {
             if path.join(".fossil").exists() {
@@ -483,6 +532,37 @@ fn mk(config: &Config, opts: &MkOptions<'_>) -> CargoResult<()> {
         }
     };
 
+    Ok(())
+}
+
+fn mk(config: &Config, opts: &MkOptions<'_>) -> CargoResult<()> {
+    let path = opts.path;
+    let name = opts.name;
+    let cfg = global_config(config)?;
+
+
+    // using the push method with two arguments ensures that the entries for
+    // both ignore and hgignore are in sync.
+    let mut ignore = IgnoreList::new();
+    ignore.push("/target", "^target/");
+    ignore.push("**/*.rs.bk", "glob:*.rs.bk\n");
+    if !opts.bin {
+        ignore.push("Cargo.lock", "glob:Cargo.lock");
+    }
+
+    let vcs = opts.version_control.unwrap_or_else(|| {
+        let in_existing_vcs = existing_vcs_repo(path.parent().unwrap_or(path), config.cwd());
+        match (cfg.version_control, in_existing_vcs) {
+            (None, false) => VersionControl::Git,
+            (Some(opt), false) => opt,
+            (_, true) => VersionControl::NoVcs,
+        }
+    });
+
+    init_vcs(path, vcs, config)?;
+    write_ignore_file(path, &ignore, vcs)?;
+
+
     let (author_name, email) = discover_author()?;
     // Hoo boy, sure glad we've got exhaustiveness checking behind us.
     let author = match (cfg.name, cfg.email, author_name, email) {
diff --git a/tests/testsuite/init.rs b/tests/testsuite/init.rs
index 378cadc89ca..4218765a4a0 100644
--- a/tests/testsuite/init.rs
+++ b/tests/testsuite/init.rs
@@ -44,6 +44,45 @@ fn simple_bin() {
         .is_file());
 }
 
+#[test]
+fn simple_git_ignore_exists() {
+    // write a .gitignore file with one entry
+    fs::create_dir_all(paths::root().join("foo")).unwrap();
+    let mut ignore_file = File::create(paths::root().join("foo/.gitignore")).unwrap();
+    ignore_file.write("/target\n**/some.file".as_bytes()).unwrap();
+
+    cargo_process("init --lib foo --edition 2015")
+        .env("USER", "foo")
+        .run();
+
+    assert!(paths::root().is_dir());
+    assert!(paths::root().join("foo/Cargo.toml").is_file());
+    assert!(paths::root().join("foo/src/lib.rs").is_file());
+    assert!(paths::root().join("foo/.git").is_dir());
+    assert!(paths::root().join("foo/.gitignore").is_file());
+
+    let fp = paths::root().join("foo/.gitignore");
+    let mut contents = String::new();
+    File::open(&fp)
+        .unwrap()
+        .read_to_string(&mut contents)
+        .unwrap();
+    assert_eq!(
+        contents,
+        "/target\n\
+        **/some.file\n\n\
+        #Added by cargo\n\
+        #\n\
+        #already existing elements are commented out\n\
+        \n\
+        #/target\n\
+        **/*.rs.bk\n\
+        Cargo.lock",
+    );
+
+    cargo_process("build").cwd(&paths::root().join("foo")).run();
+}
+
 #[test]
 fn both_lib_and_bin() {
     cargo_process("init --lib --bin")
diff --git a/tests/testsuite/new.rs b/tests/testsuite/new.rs
index ae6faa49691..7b3a814b518 100644
--- a/tests/testsuite/new.rs
+++ b/tests/testsuite/new.rs
@@ -83,9 +83,21 @@ fn simple_git() {
     assert!(paths::root().join("foo/.git").is_dir());
     assert!(paths::root().join("foo/.gitignore").is_file());
 
+    let fp = paths::root().join("foo/.gitignore");
+    let mut contents = String::new();
+    File::open(&fp)
+        .unwrap()
+        .read_to_string(&mut contents)
+        .unwrap();
+    assert_eq!(
+        contents,
+        "/target\n**/*.rs.bk\nCargo.lock",
+    );
+
     cargo_process("build").cwd(&paths::root().join("foo")).run();
 }
 
+
 #[test]
 fn no_argument() {
     cargo_process("new")