diff --git a/services/backup/src/constants.rs b/services/backup/src/constants.rs
--- a/services/backup/src/constants.rs
+++ b/services/backup/src/constants.rs
@@ -3,6 +3,7 @@
 pub const AWS_REGION: &str = "us-east-2";
 pub const MPSC_CHANNEL_BUFFER_CAPACITY: usize = 1;
 pub const ID_SEPARATOR: &str = ":";
+pub const ATTACHMENT_HOLDER_SEPARATOR: &str = ";";
 pub const RECOVERY_DATA_STR_LENGTH: usize = 20;
 
 // 400KiB limit (in docs there is KB but they mean KiB) -
diff --git a/services/backup/src/service/handlers/add_attachments.rs b/services/backup/src/service/handlers/add_attachments.rs
--- a/services/backup/src/service/handlers/add_attachments.rs
+++ b/services/backup/src/service/handlers/add_attachments.rs
@@ -1,7 +1,7 @@
 use tonic::Status;
 
 use super::proto;
-use crate::database::DatabaseClient;
+use crate::{constants::ATTACHMENT_HOLDER_SEPARATOR, database::DatabaseClient};
 
 pub async fn handle_add_attachments(
   db: &DatabaseClient,
@@ -9,3 +9,99 @@
 ) -> Result<(), Status> {
   unimplemented!()
 }
+
+/// Modifies the [`current_holders_str`] by appending attachment holders
+/// contained in [`new_holders`]. Removes duplicates. Both arguments
+/// are expected to be [`ATTACHMENT_HOLDER_SEPARATOR`] separated strings.
+fn add_new_attachments(current_holders_str: &mut String, new_holders: &str) {
+  let mut current_holders = parse_attachment_holders(current_holders_str);
+
+  let new_holders = parse_attachment_holders(new_holders);
+  current_holders.extend(new_holders);
+
+  *current_holders_str = current_holders
+    .iter()
+    .fold(String::new(), |mut value, holder| {
+      value.reserve(holder.len() + 1);
+      value.push_str(holder);
+      value.push_str(ATTACHMENT_HOLDER_SEPARATOR);
+      value
+    })
+    .trim_end_matches(ATTACHMENT_HOLDER_SEPARATOR)
+    .to_string();
+}
+
+/// Parses an [`ATTACHMENT_HOLDER_SEPARATOR`] separated string into a `HashSet`
+/// of attachment holder string slices.
+fn parse_attachment_holders(
+  holders_str: &str,
+) -> std::collections::HashSet<&str> {
+  holders_str
+    .split(ATTACHMENT_HOLDER_SEPARATOR)
+    .filter(|holder| !holder.is_empty())
+    .collect()
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+  use std::collections::HashSet;
+
+  #[test]
+  fn test_parse_attachments() {
+    let holders = "h1;h2;h3";
+    let expected = HashSet::from(["h1", "h2", "h3"]);
+    assert_eq!(parse_attachment_holders(holders), expected);
+  }
+
+  #[test]
+  fn test_empty_attachments() {
+    let actual = parse_attachment_holders("");
+    let expected = HashSet::new();
+    assert_eq!(actual, expected);
+  }
+
+  #[test]
+  fn test_add_attachments() {
+    let mut current_holders = "holder1;holder2".to_string();
+    let new_holders = "holder3;holder4";
+    add_new_attachments(&mut current_holders, new_holders);
+    assert_eq!(
+      parse_attachment_holders(&current_holders),
+      HashSet::from(["holder1", "holder2", "holder3", "holder4"])
+    );
+  }
+
+  #[test]
+  fn test_add_to_empty() {
+    let mut current_holders = String::new();
+    let new_holders = "holder3;holder4";
+    add_new_attachments(&mut current_holders, new_holders);
+    assert_eq!(
+      parse_attachment_holders(&current_holders),
+      HashSet::from(["holder3", "holder4"])
+    );
+  }
+
+  #[test]
+  fn test_add_none() {
+    let mut current_holders = "holder1;holder2".to_string();
+    let new_holders = "";
+    add_new_attachments(&mut current_holders, new_holders);
+    assert_eq!(
+      parse_attachment_holders(&current_holders),
+      HashSet::from(["holder1", "holder2"])
+    );
+  }
+
+  #[test]
+  fn test_remove_duplicates() {
+    let mut current_holders = "holder1;holder2".to_string();
+    let new_holders = "holder2;holder3";
+    add_new_attachments(&mut current_holders, new_holders);
+    assert_eq!(
+      parse_attachment_holders(&current_holders),
+      HashSet::from(["holder1", "holder2", "holder3"])
+    );
+  }
+}