diff --git a/src/repository.rs b/src/repository.rs
index 011b06f..3402ab3 100644
--- a/src/repository.rs
+++ b/src/repository.rs
@@ -1,4 +1,4 @@
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::{fmt, fs, io};
 
 use crate::error::BakareError;
@@ -10,6 +10,7 @@ use sha2::Sha512;
 use std::fmt::Formatter;
 use std::fs::File;
 use std::io::BufReader;
+use walkdir::WalkDir;
 
 /// represents a place where backup is stored an can be restored from.
 /// right now only on-disk directory storage is supported
@@ -21,6 +22,8 @@ pub struct Repository<'a> {
     index: Index,
 }
 
+const DATA_DIR_NAME: &str = "data";
+
 #[derive(Clone, Debug, PartialOrd, PartialEq, Ord, Eq, Serialize, Deserialize, Hash)]
 pub struct ItemId(Box<[u8]>);
 
@@ -95,7 +98,8 @@ impl<'a> Repository<'a> {
             return Err(BakareError::PathToStoreNotAbsolute);
         }
         let id = Repository::calculate_id(source_path)?;
-        let destination_path = self.path.join(id.to_string());
+        let destination_path = self.data_dir();
+        let destination_path = destination_path.join(id.to_string());
         let destination_path = Path::new(&destination_path);
 
         if source_path.is_file() {
@@ -126,6 +130,20 @@ impl<'a> Repository<'a> {
         }
     }
 
+    pub fn data_weight(&self) -> Result<u64, BakareError> {
+        let total_size = WalkDir::new(self.data_dir())
+            .into_iter()
+            .filter_map(|entry| entry.ok())
+            .filter_map(|entry| entry.metadata().ok())
+            .filter(|metadata| metadata.is_file())
+            .fold(0, |acc, m| acc + m.len());
+        Ok(total_size)
+    }
+
+    fn data_dir(&self) -> PathBuf {
+        self.path().join(DATA_DIR_NAME)
+    }
+
     fn calculate_id(source_path: &Path) -> Result<ItemId, BakareError> {
         let source_file = File::open(source_path)?;
         let mut reader = BufReader::new(source_file);
diff --git a/src/test/assertions.rs b/src/test/assertions.rs
index 91a85df..d3733e7 100644
--- a/src/test/assertions.rs
+++ b/src/test/assertions.rs
@@ -97,6 +97,13 @@ pub fn backup_file_with_contents(
     }
 }
 
+pub fn data_weight(repository_path: &Path) -> Result<u64, BakareError> {
+    {
+        let repository = Repository::open(repository_path)?;
+        Ok(repository.data_weight()?)
+    }
+}
+
 fn assert_directory_trees_have_same_contents(left: &Path, right: &Path) -> Result<(), BakareError> {
     let left_files = get_sorted_files_recursively(left)?;
     let right_files = get_sorted_files_recursively(right)?;
diff --git a/tests/system_tests.rs b/tests/system_tests.rs
index 8fd9818..c145eea 100644
--- a/tests/system_tests.rs
+++ b/tests/system_tests.rs
@@ -80,6 +80,28 @@ fn newer_version_should_be_greater_than_earlier_version() -> Result<(), BakareEr
     Ok(())
 }
 
+#[test]
+fn store_duplicated_files_just_once() -> Result<(), BakareError> {
+    let source = TempSource::new()?;
+    let repository_path = &tempdir()?.into_path();
+    Repository::init(repository_path)?;
+    assert_eq!(data_weight(&repository_path)?, 0);
+
+    let contents = "some contents";
+
+    backup_file_with_contents(&source, &repository_path, "1", contents)?;
+    let first_weight = data_weight(&repository_path)?;
+    assert!(first_weight > 0);
+
+    backup_file_with_contents(&source, &repository_path, "2", contents)?;
+    let second_weight = data_weight(&repository_path)?;
+    assert_eq!(first_weight, second_weight);
+
+    assert_restored_has_contents(repository_path, &source.file_path("1"), contents)?;
+    assert_restored_has_contents(repository_path, &source.file_path("2"), contents)?;
+    Ok(())
+}
+
 #[test]
 fn restore_latest_version_by_default() -> Result<(), BakareError> {
     let source = TempSource::new()?;
@@ -110,6 +132,5 @@ fn forbid_backup_of_paths_within_repository() -> Result<(), BakareError> {
 }
 
 // TODO: test concurrent writes
-// TODO: deduplicate data
 // TODO: test that index is stored separately from data
 // TODO: index corruption