diff --git a/R/slide.R b/R/slide.R
index ba5f3f3a..d94dd01a 100644
--- a/R/slide.R
+++ b/R/slide.R
@@ -186,15 +186,16 @@ epi_slide <- function(
 
   # Validate arguments
   assert_class(.x, "epi_df")
-  if (checkmate::test_class(.x, "grouped_df")) {
+  .x_orig_groups <- groups(.x)
+  if (inherits(.x, "grouped_df")) {
     expected_group_keys <- .x %>%
       key_colnames(exclude = "time_value") %>%
       sort()
     if (!identical(.x %>% group_vars() %>% sort(), expected_group_keys)) {
       cli_abort(
-        "epi_slide: `.x` must be either grouped by {expected_group_keys}. (Or you can just ungroup
-        `.x` and we'll do this grouping automatically.) You may need to aggregate your data first,
-        see aggregate_epi_df().",
+        "`.x` must be either grouped by {expected_group_keys} or ungrouped; if the latter,
+         we'll temporarily group by {expected_group_keys} for this operation. You may need
+        to aggregate your data first, see aggregate_epi_df().",
         class = "epiprocess__epi_slide__invalid_grouping"
       )
     }
@@ -300,7 +301,6 @@ epi_slide <- function(
   #   `epi_slide_one_group`.
   # - `...` from top of `epi_slide` are forwarded to `.f` here through
   #   group_modify and through the lambda.
-  .x_groups <- groups(.x)
   result <- group_map(
     .x,
     .f = function(.data_group, .group_key, ...) {
@@ -324,7 +324,7 @@ epi_slide <- function(
     filter(.real) %>%
     select(-.real) %>%
     arrange_col_canonical() %>%
-    group_by(!!!.x_groups)
+    group_by(!!!.x_orig_groups)
 
   # If every group in epi_slide_one_group takes the
   # length(available_ref_time_values) == 0 branch then we end up here.
@@ -691,12 +691,30 @@ epi_slide_opt <- function(
     )
   }
 
+  assert_class(.x, "epi_df")
+  .x_orig_groups <- groups(.x)
+  if (inherits(.x, "grouped_df")) {
+    expected_group_keys <- .x %>%
+      key_colnames(exclude = "time_value") %>%
+      sort()
+    if (!identical(.x %>% group_vars() %>% sort(), expected_group_keys)) {
+      cli_abort(
+        "`.x` must be either grouped by {expected_group_keys} or ungrouped; if the latter,
+         we'll temporarily group by {expected_group_keys} for this operation. You may need
+        to aggregate your data first, see aggregate_epi_df().",
+        class = "epiprocess__epi_slide__invalid_grouping"
+      )
+    }
+  } else {
+    .x <- group_epi_df(.x, exclude = "time_value")
+  }
   if (nrow(.x) == 0L) {
     cli_abort(
       c(
         "input data `.x` unexpectedly has 0 rows",
         "i" = "If this computation is occuring within an `epix_slide` call,
-          check that `epix_slide` `.versions` argument was set appropriately"
+          check that `epix_slide` `.versions` argument was set appropriately
+          so that you don't get any completely-empty snapshots"
       ),
       class = "epiprocess__epi_slide_opt__0_row_input",
       epiprocess__x = .x
@@ -857,27 +875,9 @@ epi_slide_opt <- function(
       arrange(.data$time_value)
 
     if (f_from_package == "data.table") {
-      # If a group contains duplicate time values, `frollmean` will still only
-      # use the last `k` obs. It isn't looking at dates, it just goes in row
-      # order. So if the computation is aggregating across multiple obs for the
-      # same date, `epi_slide_opt` and derivates will produce incorrect results;
-      # `epi_slide` should be used instead.
-      if (anyDuplicated(.data_group$time_value) != 0L) {
-        cli_abort(
-          c(
-            "group contains duplicate time values. Using `epi_slide_[opt/mean/sum]` on this
-              group will result in incorrect results",
-            "i" = "Please change the grouping structure of the input data so that
-              each group has non-duplicate time values (e.g. `x %>% group_by(geo_value)
-              %>% epi_slide_opt(.f = frollmean)`)",
-            "i" = "Use `epi_slide` to aggregate across groups"
-          ),
-          class = "epiprocess__epi_slide_opt__duplicate_time_values",
-          epiprocess__data_group = .data_group,
-          epiprocess__group_key = .group_key
-        )
-      }
-
+      # Grouping should ensure that we don't have duplicate time values.
+      # Completion above should ensure we have at least .window_size rows. Check
+      # that we don't have more than .window_size rows (or fewer somehow):
       if (nrow(.data_group) != length(c(all_dates, pad_early_dates, pad_late_dates))) {
         cli_abort(
           c(
@@ -928,7 +928,8 @@ epi_slide_opt <- function(
     group_modify(slide_one_grp, ..., .keep = FALSE) %>%
     filter(.data$.real) %>%
     select(-.real) %>%
-    arrange_col_canonical()
+    arrange_col_canonical() %>%
+    group_by(!!!.x_orig_groups)
 
   if (.all_rows) {
     result[!(result$time_value %in% ref_time_values), result_col_names] <- NA
diff --git a/man-roxygen/basic-slide-params.R b/man-roxygen/basic-slide-params.R
index 638307d6..8ccd35f9 100644
--- a/man-roxygen/basic-slide-params.R
+++ b/man-roxygen/basic-slide-params.R
@@ -1,5 +1,5 @@
-#' @param .x An `epi_df` object. If ungrouped, we group by `geo_value` and any
-#'   columns in `other_keys`. If grouped, we make sure the grouping is by
+#' @param .x An `epi_df` object. If ungrouped, we temporarily group by `geo_value`
+#'   and any columns in `other_keys`. If grouped, we make sure the grouping is by
 #'   `geo_value` and `other_keys`.
 #' @param .window_size The size of the sliding window. The accepted values
 #' depend on the type of the `time_value` column in `.x`:
diff --git a/man/epi_slide.Rd b/man/epi_slide.Rd
index c497b5d3..9f4abd36 100644
--- a/man/epi_slide.Rd
+++ b/man/epi_slide.Rd
@@ -16,8 +16,8 @@ epi_slide(
 )
 }
 \arguments{
-\item{.x}{An \code{epi_df} object. If ungrouped, we group by \code{geo_value} and any
-columns in \code{other_keys}. If grouped, we make sure the grouping is by
+\item{.x}{An \code{epi_df} object. If ungrouped, we temporarily group by \code{geo_value}
+and any columns in \code{other_keys}. If grouped, we make sure the grouping is by
 \code{geo_value} and \code{other_keys}.}
 
 \item{.f}{Function, formula, or missing; together with \code{...} specifies the
diff --git a/man/epi_slide_opt.Rd b/man/epi_slide_opt.Rd
index e61e94cd..48453641 100644
--- a/man/epi_slide_opt.Rd
+++ b/man/epi_slide_opt.Rd
@@ -47,8 +47,8 @@ epi_slide_sum(
 )
 }
 \arguments{
-\item{.x}{An \code{epi_df} object. If ungrouped, we group by \code{geo_value} and any
-columns in \code{other_keys}. If grouped, we make sure the grouping is by
+\item{.x}{An \code{epi_df} object. If ungrouped, we temporarily group by \code{geo_value}
+and any columns in \code{other_keys}. If grouped, we make sure the grouping is by
 \code{geo_value} and \code{other_keys}.}
 
 \item{.col_names}{<\code{\link[=dplyr_tidy_select]{tidy-select}}> An unquoted column
diff --git a/tests/testthat/test-epi_slide.R b/tests/testthat/test-epi_slide.R
index 2cb04eec..0aa4aca7 100644
--- a/tests/testthat/test-epi_slide.R
+++ b/tests/testthat/test-epi_slide.R
@@ -899,3 +899,41 @@ test_that("epi_slide_opt output naming features", {
     class = "epiprocess__epi_slide_opt_new_name_duplicated"
   )
 })
+
+test_that("epi_slide* output grouping matches input grouping", {
+  toy_edf <- as_epi_df(bind_rows(list(
+    tibble(geo_value = 1, age_group = 1, time_value = as.Date("2020-01-01") + 1:10 - 1, value = 1:10),
+    tibble(geo_value = 1, age_group = 2, time_value = as.Date("2020-01-01") + 1:10 - 1, value = 20:11),
+    tibble(geo_value = 2, age_group = 2, time_value = as.Date("2020-01-01") + 1:10 - 1, value = 31:40)
+  )), other_keys = "age_group", as_of = as.Date("2020-01-01") + 20)
+
+  # Preserving existing grouping:
+  expect_equal(
+    toy_edf %>%
+      group_by(age_group, geo_value) %>%
+      epi_slide(value_7dsum = sum(value), .window_size = 7) %>%
+      group_vars(),
+    c("age_group", "geo_value")
+  )
+  expect_equal(
+    toy_edf %>%
+      group_by(age_group, geo_value) %>%
+      epi_slide_sum(value, .window_size = 7) %>%
+      group_vars(),
+    c("age_group", "geo_value")
+  )
+
+  # Removing automatic grouping:
+  expect_equal(
+    toy_edf %>%
+      epi_slide(value_7dsum = sum(value), .window_size = 7) %>%
+      group_vars(),
+    character(0)
+  )
+  expect_equal(
+    toy_edf %>%
+      epi_slide_sum(value, .window_size = 7) %>%
+      group_vars(),
+    character(0)
+  )
+})