diff --git a/DESCRIPTION b/DESCRIPTION
index 895647f..74fd3af 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -14,11 +14,15 @@ Imports:
     magrittr,
     purrr,
     stringr
-Suggests: 
+Suggests:
+    jsonlite,
     knitr,
-    rmarkdown
+    rmarkdown,
+    testthat (>= 3.0.0),
+    tibble
+Config/testthat/edition: 3
 VignetteBuilder: 
     knitr
 Encoding: UTF-8
 LazyData: true
-RoxygenNote: 7.2.3
+RoxygenNote: 7.3.3
diff --git a/NAMESPACE b/NAMESPACE
index a428cef..cc9d3f7 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -9,6 +9,7 @@ export(get_all_bookmarks_from_channel)
 export(get_all_bookmarks_id_from_channel)
 export(get_channel_id)
 export(get_user_id)
+export(get_users_in_channel)
 export(invite_users_to_channel)
 export(remove_all_bookmarks_from_channel)
 export(remove_bookmark_starting_with)
diff --git a/R/functions.R b/R/functions.R
index e8836dd..bc62ee4 100644
--- a/R/functions.R
+++ b/R/functions.R
@@ -38,3 +38,40 @@ get_user_id <- function(name,users = slackr::slackr_users()){
 get_id <- function(name,from){
   from %>% select(name,id) %>% filter(name %in% !!name) %>% pull(id)
 }
+
+#' List the user ids already in a Slack channel
+#'
+#' Wraps the `conversations.members` Slack API endpoint. Used by
+#' [invite_users_to_channel()] to skip users who are already in the
+#' target channel.
+#'
+#' @param channel channel name (case-insensitive).
+#' @param token slack api token.
+#' @param all_channel cached `slackr::slackr_channels()` result, passed
+#'   through to [get_channel_id()].
+#'
+#' @return character vector of Slack user ids. Empty when the API call
+#'   fails or the channel has no members.
+#' @export
+#'
+#' @importFrom httr POST content
+get_users_in_channel <- function(channel,
+                                 token = Sys.getenv("SLACK_API_TOKEN"),
+                                 all_channel = slackr::slackr_channels()) {
+  channel_id <- get_channel_id(name = tolower(channel), all_channel = all_channel)
+  if (length(channel_id) == 0L) return(character())
+
+  resp <- tryCatch(
+    httr::POST(
+      url = "https://slack.com/api/conversations.members",
+      body = list(token = token, channel = channel_id)
+    ),
+    error = function(e) NULL
+  )
+  if (is.null(resp)) return(character())
+
+  body <- httr::content(resp)
+  if (!isTRUE(body$ok)) return(character())
+  members <- unlist(body$members, use.names = FALSE)
+  if (is.null(members)) character() else as.character(members)
+}
diff --git a/R/invite_user_to_channel.R b/R/invite_user_to_channel.R
index adf8d69..1717e61 100644
--- a/R/invite_user_to_channel.R
+++ b/R/invite_user_to_channel.R
@@ -19,10 +19,25 @@ invite_users_to_channel <- function(channel, users, token=Sys.getenv("SLACK_API_
     # Sys.sleep(1)
   }
 
+  # Skip the current user (would be a self-invite error) AND anyone
+  # already in the channel (would otherwise produce a noisy
+  # 'already_in_channel' error and re-send a notification).
+  already_in <- get_users_in_channel(
+    channel = channel, token = token, all_channel = all_channel
+  )
+  to_invite <- setdiff(get_user_id(users), c(current_user, already_in))
+
+  if (length(to_invite) == 0L) {
+    message(
+      "Nothing to do: all requested users are already in #", channel, "."
+    )
+    return(invisible(channel))
+  }
+
   httr::POST(url="https://slack.com/api/conversations.invite",
              body=list( token=token,
                         channel= get_channel_id(name = tolower(channel),all_channel = all_channel),
-                        users=paste(     setdiff(  get_user_id(users),current_user )  ,collapse=",")))
+                        users=paste(to_invite, collapse=",")))
   # Sys.sleep(1)
   invisible(channel)
 }
diff --git a/man/get_users_in_channel.Rd b/man/get_users_in_channel.Rd
new file mode 100644
index 0000000..32f3d74
--- /dev/null
+++ b/man/get_users_in_channel.Rd
@@ -0,0 +1,29 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/functions.R
+\name{get_users_in_channel}
+\alias{get_users_in_channel}
+\title{List the user ids already in a Slack channel}
+\usage{
+get_users_in_channel(
+  channel,
+  token = Sys.getenv("SLACK_API_TOKEN"),
+  all_channel = slackr::slackr_channels()
+)
+}
+\arguments{
+\item{channel}{channel name (case-insensitive).}
+
+\item{token}{slack api token.}
+
+\item{all_channel}{cached `slackr::slackr_channels()` result, passed
+through to [get_channel_id()].}
+}
+\value{
+character vector of Slack user ids. Empty when the API call
+  fails or the channel has no members.
+}
+\description{
+Wraps the `conversations.members` Slack API endpoint. Used by
+[invite_users_to_channel()] to skip users who are already in the
+target channel.
+}
diff --git a/man/invite_users_to_channel.Rd b/man/invite_users_to_channel.Rd
index 9f171f1..8e4a1cd 100644
--- a/man/invite_users_to_channel.Rd
+++ b/man/invite_users_to_channel.Rd
@@ -9,7 +9,8 @@ invite_users_to_channel(
   users,
   token = Sys.getenv("SLACK_API_TOKEN"),
   create_channel = TRUE,
-  all_channel = slackr::slackr_channels()
+  all_channel = slackr::slackr_channels(),
+  current_user = slackr::auth_test()$user_id
 )
 }
 \arguments{
@@ -20,6 +21,8 @@ invite_users_to_channel(
 \item{token}{slack api token}
 
 \item{create_channel}{boolean create chanel if needed}
+
+\item{current_user}{current user id}
 }
 \description{
 Add user(s) to a channel
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 0000000..0cf3c42
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,4 @@
+library(testthat)
+library(slack)
+
+test_check("slack")
diff --git a/tests/testthat/test-invite_users_to_channel.R b/tests/testthat/test-invite_users_to_channel.R
new file mode 100644
index 0000000..6db750f
--- /dev/null
+++ b/tests/testthat/test-invite_users_to_channel.R
@@ -0,0 +1,118 @@
+# Mock-based tests for invite_users_to_channel + get_users_in_channel.
+# We don't talk to the real Slack API; we stub httr::POST and the slackr
+# helpers and inspect what would have been sent.
+
+fake_response <- function(content, ok = TRUE) {
+  body <- c(list(ok = ok), content)
+  enc  <- charToRaw(jsonlite::toJSON(body, auto_unbox = TRUE))
+  structure(
+    list(
+      status_code = 200L,
+      headers = structure(list(`content-type` = "application/json"),
+                          class = "insensitive"),
+      content = enc
+    ),
+    class = "response"
+  )
+}
+
+# A tibble mimicking slackr::slackr_users() / slackr_channels() shape.
+fake_users <- function() {
+  tibble::tibble(name = c("alice", "bob", "carol"),
+                 id   = c("U_ALICE", "U_BOB", "U_CAROL"))
+}
+fake_channels <- function() {
+  tibble::tibble(name = c("general", "team-r"),
+                 id   = c("C_GEN", "C_R"))
+}
+
+# Whatever httr::POST stub the test installs returns this — recorded so
+# the test can inspect both endpoints (.members, .invite).
+post_log <- new.env(parent = emptyenv())
+
+with_slack_mocks <- function(members_in_channel, code) {
+  post_log$calls <- list()
+  fake_post <- function(url, body = NULL, ...) {
+    post_log$calls <- c(post_log$calls, list(list(url = url, body = body)))
+    if (grepl("conversations.members$", url)) {
+      return(fake_response(list(members = as.list(members_in_channel))))
+    }
+    if (grepl("conversations.invite$", url)) {
+      return(fake_response(list(channel = list(id = body$channel))))
+    }
+    fake_response(list())
+  }
+  testthat::with_mocked_bindings(
+    POST = fake_post,
+    .package = "httr",
+    {
+      testthat::with_mocked_bindings(
+        slackr_users    = function(...) fake_users(),
+        slackr_channels = function(...) fake_channels(),
+        auth_test       = function(...) list(user_id = "U_ALICE"),
+        .package = "slackr",
+        code
+      )
+    }
+  )
+}
+
+test_that("get_users_in_channel returns the parsed members list", {
+  with_slack_mocks(
+    members_in_channel = c("U_ALICE", "U_BOB"),
+    code = {
+      ids <- get_users_in_channel("general", token = "fake")
+      expect_equal(sort(ids), c("U_ALICE", "U_BOB"))
+    }
+  )
+})
+
+test_that("get_users_in_channel returns character() when the channel is unknown", {
+  with_slack_mocks(
+    members_in_channel = character(),
+    code = {
+      ids <- get_users_in_channel("does-not-exist", token = "fake")
+      expect_equal(ids, character())
+    }
+  )
+})
+
+test_that("invite_users_to_channel skips users already in the channel", {
+  with_slack_mocks(
+    members_in_channel = c("U_ALICE", "U_BOB"),
+    code = {
+      invite_users_to_channel(
+        channel = "general",
+        users   = c("alice", "bob", "carol"),
+        token   = "fake",
+        create_channel = FALSE
+      )
+    }
+  )
+  invite_call <- Filter(
+    function(x) grepl("invite$", x$url),
+    post_log$calls
+  )
+  expect_length(invite_call, 1L)
+  expect_equal(invite_call[[1]]$body$users, "U_CAROL")
+})
+
+test_that("invite_users_to_channel does nothing when nobody is to add", {
+  with_slack_mocks(
+    members_in_channel = c("U_ALICE", "U_BOB", "U_CAROL"),
+    code = {
+      msg <- capture_messages(invite_users_to_channel(
+        channel = "general",
+        users   = c("alice", "bob", "carol"),
+        token   = "fake",
+        create_channel = FALSE
+      ))
+      expect_match(paste(msg, collapse = " "), "Nothing to do", fixed = TRUE)
+    }
+  )
+  invite_call <- Filter(
+    function(x) grepl("invite$", x$url),
+    post_log$calls
+  )
+  expect_length(invite_call, 0L)
+})