fix(crypto): Only send cancellations if there were multiple request recipients

crates/matrix-sdk-crypto/src/verification/requests.rs

@@ -532,12 +532,20 @@ impl VerificationRequest {
                 .filter(|d| if let Some(device) = filter_device { &**d != device } else { true })
                 .collect();
 
+            // We don't need to notify anyone if no recipients were present
+            // but we did have a filter device, since this means that only a
+            // single device received the `m.key.verification.request` and that
+            // device accepted the request.
+            if recipients.is_empty() && filter_device.is_some() {
+                None
+            } else {
                 Some(ToDeviceRequest::new_for_recipients(
                     self.other_user(),
                     recipients,
                     c,
                     Uuid::new_v4(),
                 ))
+            }
         } else {
             None
         }