api4/user: Deduplicate userids for getUsersByIds endpoint (#24344)

server/channels/api4/user_test.go

@@ -1725,6 +1725,13 @@ func TestGetUsersByIds(t *testing.T) {
 
 			require.Len(t, users, 1, "1 user should be returned")
 		})
+
+		t.Run("should only return unique users when multiple IDs are requested", func(t *testing.T) {
+			users, _, err := client.GetUsersByIds(context.Background(), []string{th.BasicUser.Id, th.BasicUser.Id, th.BasicUser.Id})
+			require.NoError(t, err)
+
+			require.Len(t, users, 1, "1 user should be returned")
+		})
 	})
 
 	t.Run("should return error when not logged in", func(t *testing.T) {