diff --git a/R/dplyr.R b/R/dplyr.R
index dd1129355..7d4fd7c7e 100644
--- a/R/dplyr.R
+++ b/R/dplyr.R
@@ -80,9 +80,11 @@ rows_check_y_unmatched <- dplyr$rows_check_y_unmatched
 rows_df_in_place <- dplyr$rows_df_in_place
 rowwise_df <- dplyr$rowwise_df
 slice_rows <- dplyr$slice_rows
+stop_join <- dplyr$stop_join
 summarise_build <- dplyr$summarise_build
 summarise_cols <- dplyr$summarise_cols
 summarise_deprecate_variable_size <- dplyr$summarise_deprecate_variable_size
 the <- dplyr$the
 tick_if_needed <- dplyr$tick_if_needed
+warn_join <- dplyr$warn_join
 warn_join_cross_by <- dplyr$warn_join_cross_by
diff --git a/R/full_join.R b/R/full_join.R
index 40578e11a..12d6469f3 100644
--- a/R/full_join.R
+++ b/R/full_join.R
@@ -17,7 +17,7 @@ full_join.duckplyr_df <- function(x, y, by = NULL, copy = FALSE, suffix = c(".x"
     "No implicit cross joins for {.code full_join()}" = is_cross_by(by),
     "{.arg multiple} not supported" = !identical(multiple, "all"),
     {
-      out <- rel_join_impl(x, y, by, "full", na_matches, suffix, keep, error_call)
+      out <- rel_join_impl(x, y, by, "full", na_matches, suffix, keep, relationship, error_call)
       return(out)
     }
   )
diff --git a/R/inner_join.R b/R/inner_join.R
index 29bc68699..45e1bd058 100644
--- a/R/inner_join.R
+++ b/R/inner_join.R
@@ -19,7 +19,7 @@ inner_join.duckplyr_df <- function(x, y, by = NULL, copy = FALSE, suffix = c(".x
     "{.arg multiple} not supported" = !identical(multiple, "all"),
     "{.arg unmatched} not supported" = !identical(unmatched, "drop"),
     {
-      out <- rel_join_impl(x, y, by, "inner", na_matches, suffix, keep, error_call)
+      out <- rel_join_impl(x, y, by, "inner", na_matches, suffix, keep, relationship, error_call)
       return(out)
     }
   )
diff --git a/R/join.R b/R/join.R
index 42dde6927..ceaf65581 100644
--- a/R/join.R
+++ b/R/join.R
@@ -6,6 +6,7 @@ rel_join_impl <- function(
   na_matches,
   suffix = c(".x", ".y"),
   keep = NULL,
+  relationship = NULL,
   error_call = caller_env()
 ) {
   mutating <- !(join %in% c("semi", "anti"))
@@ -25,6 +26,10 @@ rel_join_impl <- function(
     by <- as_join_by(by, error_call = error_call)
   }
 
+  if (mutating) {
+    check_relationship(relationship, x, y, by, error_call = error_call)
+  }
+
   x_by <- by$x
   y_by <- by$y
   x_rel <- duckdb_rel_from_df(x)
@@ -136,3 +141,62 @@ rel_join_impl <- function(
 
   return(out)
 }
+
+check_relationship <- function(relationship, x, y, by, error_call) {
+  if (is_null(relationship)) {
+    # FIXME: Determine behavior based on option
+    if (!is_key(x, by$x) && !is_key(y, by$y)) {
+      warn_join(
+        message = c(
+          "Detected an unexpected many-to-many relationship between `x` and `y`.",
+          i = paste0(
+            "If a many-to-many relationship is expected, ",
+            "set `relationship = \"many-to-many\"` to silence this warning."
+          )
+        ),
+        class = "dplyr_warning_join_relationship_many_to_many",
+        call = error_call
+      )
+    }
+    return()
+  }
+
+  if (relationship %in% c("one-to-many", "one-to-one")) {
+    if (!is_key(x, by$x)) {
+      stop_join(
+        message = c(
+          glue("Each row in `{x_name}` must match at most 1 row in `{y_name}`."),
+        ),
+        class = paste0("dplyr_error_join_relationship_", gsub("-", "_", relationship)),
+        call = error_call
+      )
+    }
+  }
+
+  if (relationship %in% c("many-to-one", "one-to-one")) {
+    if (!is_key(y, by$y)) {
+      stop_join(
+        message = c(
+          glue("Each row in `{y_name}` must match at most 1 row in `{x_name}`."),
+        ),
+        class = paste0("dplyr_error_join_relationship_", gsub("-", "_", relationship)),
+        call = error_call
+      )
+    }
+  }
+}
+
+is_key <- function(x, cols) {
+  local_options(duckdb.materialize_message = FALSE)
+
+  rows <-
+    x %>%
+    # FIXME: Why does this materialize
+    # as_duckplyr_tibble() %>%
+    summarize(.by = c(!!!syms(cols)), `___n` = n()) %>%
+    filter(`___n` > 1L) %>%
+    head(1L) %>%
+    nrow()
+
+  rows == 0
+}
diff --git a/R/left_join.R b/R/left_join.R
index f171a8054..565b3f375 100644
--- a/R/left_join.R
+++ b/R/left_join.R
@@ -20,7 +20,7 @@ left_join.duckplyr_df <- function(x, y, by = NULL, copy = FALSE, suffix = c(".x"
     "{.arg multiple} not supported" = !identical(multiple, "all"),
     "{.arg unmatched} not supported" = !identical(unmatched, "drop"),
     {
-      out <- rel_join_impl(x, y, by, "left", na_matches, suffix, keep, error_call)
+      out <- rel_join_impl(x, y, by, "left", na_matches, suffix, keep, relationship, error_call)
       return(out)
     }
   )
diff --git a/R/relational-duckdb.R b/R/relational-duckdb.R
index 95a301cb3..74ee1d14c 100644
--- a/R/relational-duckdb.R
+++ b/R/relational-duckdb.R
@@ -33,7 +33,7 @@ duckplyr_macros <- c(
   #
   "___divide" = "(x, y) AS CASE WHEN y = 0 THEN CASE WHEN x = 0 THEN CAST('NaN' AS double) WHEN x > 0 THEN CAST('+Infinity' AS double) ELSE CAST('-Infinity' AS double) END ELSE CAST(x AS double) / y END",
   #
-  "is.na" = "(x) AS (x IS NULL)",
+  "is.na" = "(x) AS (x IS NULL OR isnan(x))",
   "n" = "() AS CAST(COUNT(*) AS int32)",
   #
   "___log10" = "(x) AS CASE WHEN x < 0 THEN CAST('NaN' AS double) WHEN x = 0 THEN CAST('-Inf' AS double) ELSE log10(x) END",
@@ -181,6 +181,14 @@ check_df_for_rel <- function(df, call = caller_env()) {
       if (!identical(df_attrib, roundtrip_attrib)) {
         cli::cli_abort("Attributes are lost during conversion. Affected column: {.var {names(df)[[i]]}}.", call = call)
       }
+      # Always check roundtrip for timestamp columns
+      # duckdb uses microsecond precision only, this is in some cases
+      # less than R does
+      if (inherits(df[[i]], "POSIXct")) {
+        if (!identical(df[[i]], roundtrip[[i]])) {
+          cli::cli_abort("Imperfect roundtrip. Affected column: {.var {names(df)[[i]]}}.", call = call)
+        }
+      }
     }
   }
 
diff --git a/R/right_join.R b/R/right_join.R
index 38ed32b76..5d97298a3 100644
--- a/R/right_join.R
+++ b/R/right_join.R
@@ -19,7 +19,7 @@ right_join.duckplyr_df <- function(x, y, by = NULL, copy = FALSE, suffix = c(".x
     "{.arg multiple} not supported" = !identical(multiple, "all"),
     "{.arg unmatched} not supported" = !identical(unmatched, "drop"),
     {
-      out <- rel_join_impl(x, y, by, "right", na_matches, suffix, keep, error_call)
+      out <- rel_join_impl(x, y, by, "right", na_matches, suffix, keep, relationship, error_call)
       return(out)
     }
   )
diff --git a/tests/testthat/_snaps/dplyr-join-rows.md b/tests/testthat/_snaps/dplyr-join-rows.md
index 07d31a943..f478fd51f 100644
--- a/tests/testthat/_snaps/dplyr-join-rows.md
+++ b/tests/testthat/_snaps/dplyr-join-rows.md
@@ -65,6 +65,39 @@
       ! Each row in `x` must match at most 1 row in `y`.
       i Row 1 of `x` matches multiple rows in `y`.
 
+# join_rows() gives meaningful many-to-many warnings
+
+    Code
+      join_rows(c(1, 1), c(1, 1))
+    Condition
+      Warning:
+      Detected an unexpected many-to-many relationship between `x` and `y`.
+      i Row 1 of `x` matches multiple rows in `y`.
+      i Row 1 of `y` matches multiple rows in `x`.
+      i If a many-to-many relationship is expected, set `relationship = "many-to-many"` to silence this warning.
+    Output
+      $x
+      [1] 1 1 2 2
+      
+      $y
+      [1] 1 2 1 2
+      
+
+---
+
+    Code
+      duckplyr_left_join(df, df, by = join_by(x))
+    Condition
+      Warning in `duckplyr_left_join()`:
+      Detected an unexpected many-to-many relationship between `x` and `y`.
+      i If a many-to-many relationship is expected, set `relationship = "many-to-many"` to silence this warning.
+    Output
+        x
+      1 1
+      2 1
+      3 1
+      4 1
+
 # join_rows() gives meaningful error message on unmatched rows
 
     Code
diff --git a/tests/testthat/_snaps/dplyr-join.md b/tests/testthat/_snaps/dplyr-join.md
index 73111ec5a..28ebdbb85 100644
--- a/tests/testthat/_snaps/dplyr-join.md
+++ b/tests/testthat/_snaps/dplyr-join.md
@@ -50,6 +50,15 @@
       Error:
       ! `na_matches` must be one of "na" or "never", not "foo".
 
+# mutating joins trigger many-to-many warning
+
+    Code
+      out <- duckplyr_left_join(df, df, join_by(x))
+    Condition
+      Warning in `duckplyr_left_join()`:
+      Detected an unexpected many-to-many relationship between `x` and `y`.
+      i If a many-to-many relationship is expected, set `relationship = "many-to-many"` to silence this warning.
+
 # mutating joins compute common columns
 
     Code
diff --git a/tests/testthat/test-dplyr-join-rows.R b/tests/testthat/test-dplyr-join-rows.R
index 3cd8ff956..cd7edf110 100644
--- a/tests/testthat/test-dplyr-join-rows.R
+++ b/tests/testthat/test-dplyr-join-rows.R
@@ -197,7 +197,6 @@ test_that("join_rows() gives meaningful many-to-one errors", {
 })
 
 test_that("join_rows() gives meaningful many-to-many warnings", {
-  skip("TODO duckdb")
   expect_snapshot({
     join_rows(c(1, 1), c(1, 1))
   })
diff --git a/tests/testthat/test-dplyr-join.R b/tests/testthat/test-dplyr-join.R
index 60b8355ef..f66be8a5b 100644
--- a/tests/testthat/test-dplyr-join.R
+++ b/tests/testthat/test-dplyr-join.R
@@ -360,7 +360,6 @@ test_that("join_filter() validates arguments", {
 })
 
 test_that("mutating joins trigger many-to-many warning", {
-  skip("TODO duckdb")
   df <- tibble(x = c(1, 1))
   expect_snapshot(out <- duckplyr_left_join(df, df, join_by(x)))
 })
diff --git a/tools/00-funs.R b/tools/00-funs.R
index 2ecf6094e..b1b3fe493 100644
--- a/tools/00-funs.R
+++ b/tools/00-funs.R
@@ -237,14 +237,10 @@ duckplyr_tests <- head(n = -1, list(
     NULL
   ),
   "test-join-rows.R" = c(
-    "join_rows() gives meaningful many-to-many warnings",
     NULL
   ),
   "test-join.R" = c(
-    "mutating joins trigger multiple match warning",
-    "mutating joins don't trigger multiple match warning when called indirectly",
-
-    "mutating joins trigger many-to-many warning",
+    # FIXME: How to detect an indirect call?
     "mutating joins don't trigger many-to-many warning when called indirectly",
     NULL
   ),