diff --git a/Cargo.lock b/Cargo.lock
index b41629ecf..ab59bcd7c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2944,6 +2944,7 @@ dependencies = [
  "futures-util",
  "imbl",
  "indexmap",
+ "itertools",
  "matrix-sdk",
  "matrix-sdk-base",
  "matrix-sdk-crypto",
diff --git a/Cargo.toml b/Cargo.toml
index aec8cd99f..f7bcffc0a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -35,6 +35,7 @@ futures-core = "0.3.28"
 futures-executor = "0.3.21"
 futures-util = { version = "0.3.26", default-features = false, features = ["alloc"] }
 http = "0.2.6"
+itertools = "0.10.5"
 ruma = { git = "https://github.com/ruma/ruma", rev = "cf32036df4c9daca736dcd7f0d9d65debcf9897f", features = ["client-api-c", "compat-user-id"] }
 ruma-common = { git = "https://github.com/ruma/ruma", rev = "cf32036df4c9daca736dcd7f0d9d65debcf9897f" }
 once_cell = "1.16.0"
diff --git a/crates/matrix-sdk-crypto/Cargo.toml b/crates/matrix-sdk-crypto/Cargo.toml
index cc155eb60..5b06879ad 100644
--- a/crates/matrix-sdk-crypto/Cargo.toml
+++ b/crates/matrix-sdk-crypto/Cargo.toml
@@ -44,7 +44,7 @@ futures-util = { workspace = true }
 hkdf = { version = "0.12.3", optional = true }
 hmac = "0.12.1"
 http = { workspace = true, optional = true } # feature = testing only
-itertools = "0.10.5"
+itertools = { workspace = true }
 matrix-sdk-qrcode = { version = "0.4.0", path = "../matrix-sdk-qrcode", optional = true }
 matrix-sdk-common = { version = "0.6.0", path = "../matrix-sdk-common" }
 pbkdf2 = { version = "0.11.0", default-features = false }
diff --git a/crates/matrix-sdk-ui/Cargo.toml b/crates/matrix-sdk-ui/Cargo.toml
index 6ee0eeeec..2fb8578bd 100644
--- a/crates/matrix-sdk-ui/Cargo.toml
+++ b/crates/matrix-sdk-ui/Cargo.toml
@@ -30,6 +30,7 @@ futures-core = { workspace = true }
 futures-util = { workspace = true }
 imbl = { version = "2.0.0", features = ["serde"] }
 indexmap = "1.9.1"
+itertools = { workspace = true }
 matrix-sdk = { version = "0.6.2", path = "../matrix-sdk", default-features = false }
 matrix-sdk-base = { version = "0.6.1", path = "../matrix-sdk-base" }
 matrix-sdk-crypto = { version = "0.6.0", path = "../matrix-sdk-crypto" }
diff --git a/crates/matrix-sdk-ui/src/timeline/event_item/content.rs b/crates/matrix-sdk-ui/src/timeline/event_item/content.rs
index 921c92d33..4acdbc842 100644
--- a/crates/matrix-sdk-ui/src/timeline/event_item/content.rs
+++ b/crates/matrix-sdk-ui/src/timeline/event_item/content.rs
@@ -16,6 +16,7 @@ use std::{fmt, ops::Deref, sync::Arc};
 
 use imbl::{vector, Vector};
 use indexmap::IndexMap;
+use itertools::Itertools;
 use matrix_sdk::{deserialized_responses::TimelineEvent, Result};
 use ruma::{
     assign,
@@ -398,9 +399,9 @@ type ReactionGroupInner = IndexMap<(Option<OwnedTransactionId>, Option<OwnedEven
 pub struct ReactionGroup(pub(in crate::timeline) ReactionGroupInner);
 
 impl ReactionGroup {
-    /// The senders of the reactions in this group.
+    /// The (deduplicated) senders of the reactions in this group.
     pub fn senders(&self) -> impl Iterator<Item = &UserId> {
-        self.values().map(AsRef::as_ref)
+        self.values().unique().map(AsRef::as_ref)
     }
 }
 
diff --git a/crates/matrix-sdk-ui/src/timeline/tests/mod.rs b/crates/matrix-sdk-ui/src/timeline/tests/mod.rs
index 6da4bb951..7079dabf6 100644
--- a/crates/matrix-sdk-ui/src/timeline/tests/mod.rs
+++ b/crates/matrix-sdk-ui/src/timeline/tests/mod.rs
@@ -53,6 +53,7 @@ mod edit;
 #[cfg(feature = "e2e-encryption")]
 mod encryption;
 mod invalid;
+mod reaction_group;
 mod read_receipts;
 mod redaction;
 mod virt;
diff --git a/crates/matrix-sdk-ui/src/timeline/tests/reaction_group.rs b/crates/matrix-sdk-ui/src/timeline/tests/reaction_group.rs
new file mode 100644
index 000000000..98c4077d7
--- /dev/null
+++ b/crates/matrix-sdk-ui/src/timeline/tests/reaction_group.rs
@@ -0,0 +1,41 @@
+// Copyright 2023 The Matrix.org Foundation C.I.C.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use ruma::{server_name, EventId, UserId};
+
+use crate::timeline::{
+    tests::{ALICE, BOB},
+    ReactionGroup,
+};
+
+/// The Matrix spec does not allow duplicate annotations to be created but it
+/// is still possible for duplicates to be received over federation. And in
+/// that case, clients are expected to treat duplicates as a single annotation.
+#[test]
+fn senders_are_deduplicated() {
+    let group = {
+        let mut group = ReactionGroup::default();
+        insert(&mut group, &ALICE, 3);
+        insert(&mut group, &BOB, 2);
+        group
+    };
+
+    assert_eq!(group.senders().collect::<Vec<_>>(), vec![&ALICE.to_owned(), &BOB.to_owned()]);
+}
+
+fn insert(group: &mut ReactionGroup, sender: &UserId, count: u64) {
+    for _ in 0..count {
+        let event_id = EventId::new(server_name!("dummy.server"));
+        group.0.insert((None, Some(event_id)), sender.to_owned());
+    }
+}