diff --git a/parallel-checkout.c b/parallel-checkout.c
index 09e8b10a35..6fb3f1e6c9 100644
--- a/parallel-checkout.c
+++ b/parallel-checkout.c
@@ -8,6 +8,7 @@
 #include "sigchain.h"
 #include "streaming.h"
 #include "thread-utils.h"
+#include "trace2.h"
 
 struct pc_worker {
 	struct child_process cp;
@@ -326,6 +327,7 @@ void write_pc_item(struct parallel_checkout_item *pc_item,
 	if (dir_sep && !has_dirs_only_path(path.buf, dir_sep - path.buf,
 					   state->base_dir_len)) {
 		pc_item->status = PC_ITEM_COLLIDED;
+		trace2_data_string("pcheckout", NULL, "collision/dirname", path.buf);
 		goto out;
 	}
 
@@ -341,6 +343,8 @@ void write_pc_item(struct parallel_checkout_item *pc_item,
 			 * call should have already caught these cases.
 			 */
 			pc_item->status = PC_ITEM_COLLIDED;
+			trace2_data_string("pcheckout", NULL,
+					   "collision/basename", path.buf);
 		} else {
 			error_errno("failed to open file '%s'", path.buf);
 			pc_item->status = PC_ITEM_FAILED;
diff --git a/t/lib-parallel-checkout.sh b/t/lib-parallel-checkout.sh
index 39fd36fdf6..16ee18389b 100644
--- a/t/lib-parallel-checkout.sh
+++ b/t/lib-parallel-checkout.sh
@@ -21,12 +21,12 @@ test_checkout_workers () {
 	shift &&
 
 	rm -f trace &&
-	GIT_TRACE2="$(pwd)/trace" "$@" &&
+	GIT_TRACE2="$(pwd)/trace" "$@" 2>&8 &&
 
 	workers=$(grep "child_start\[..*\] git checkout--worker" trace | wc -l) &&
 	test $workers -eq $expected_workers &&
 	rm -f trace
-}
+} 8>&2 2>&4
 
 # Verify that both the working tree and the index were created correctly
 verify_checkout () {
diff --git a/t/t2081-parallel-checkout-collisions.sh b/t/t2081-parallel-checkout-collisions.sh
new file mode 100755
index 0000000000..f6fcfc0c1e
--- /dev/null
+++ b/t/t2081-parallel-checkout-collisions.sh
@@ -0,0 +1,162 @@
+#!/bin/sh
+
+test_description="path collisions during parallel checkout
+
+Parallel checkout must detect path collisions to:
+
+1) Avoid racily writing to different paths that represent the same file on disk.
+2) Report the colliding entries on clone.
+
+The tests in this file exercise parallel checkout's collision detection code in
+both these mechanics.
+"
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-parallel-checkout.sh"
+
+TEST_ROOT="$PWD"
+
+test_expect_success CASE_INSENSITIVE_FS 'setup' '
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+	cat >objs <<-EOF &&
+	100644 $empty_oid	FILE_X
+	100644 $empty_oid	FILE_x
+	100644 $empty_oid	file_X
+	100644 $empty_oid	file_x
+	EOF
+	git update-index --index-info <objs &&
+	git commit -m "colliding files" &&
+	git tag basename_collision &&
+
+	write_script "$TEST_ROOT"/logger_script <<-\EOF
+	echo "$@" >>filter.log
+	EOF
+'
+
+test_workers_in_event_trace ()
+{
+	test $1 -eq $(grep ".event.:.child_start..*checkout--worker" $2 | wc -l)
+}
+
+test_expect_success CASE_INSENSITIVE_FS 'worker detects basename collision' '
+	GIT_TRACE2_EVENT="$(pwd)/trace" git \
+		-c checkout.workers=2 -c checkout.thresholdForParallelism=0 \
+		checkout . &&
+
+	test_workers_in_event_trace 2 trace &&
+	collisions=$(grep -i "category.:.pcheckout.,.key.:.collision/basename.,.value.:.file_x.}" trace | wc -l) &&
+	test $collisions -eq 3
+'
+
+test_expect_success CASE_INSENSITIVE_FS 'worker detects dirname collision' '
+	test_config filter.logger.smudge "\"$TEST_ROOT/logger_script\" %f" &&
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+
+	# By setting a filter command to "a", we make it ineligible for parallel
+	# checkout, and thus it is checked out *first*. This way we can ensure
+	# that "A/B" and "A/C" will both collide with the regular file "a".
+	#
+	attr_oid=$(echo "a filter=logger" | git hash-object -w --stdin) &&
+
+	cat >objs <<-EOF &&
+	100644 $empty_oid	A/B
+	100644 $empty_oid	A/C
+	100644 $empty_oid	a
+	100644 $attr_oid	.gitattributes
+	EOF
+	git rm -rf . &&
+	git update-index --index-info <objs &&
+
+	rm -f trace filter.log &&
+	GIT_TRACE2_EVENT="$(pwd)/trace" git \
+		-c checkout.workers=2 -c checkout.thresholdForParallelism=0 \
+		checkout . &&
+
+	# Check that "a" (and only "a") was filtered
+	echo a >expected.log &&
+	test_cmp filter.log expected.log &&
+
+	# Check that it used the right number of workers and detected the collisions
+	test_workers_in_event_trace 2 trace &&
+	grep "category.:.pcheckout.,.key.:.collision/dirname.,.value.:.A/B.}" trace &&
+	grep "category.:.pcheckout.,.key.:.collision/dirname.,.value.:.A/C.}" trace
+'
+
+test_expect_success SYMLINKS,CASE_INSENSITIVE_FS 'do not follow symlinks colliding with leading dir' '
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+	symlink_oid=$(echo "./e" | git hash-object -w --stdin) &&
+	mkdir e &&
+
+	cat >objs <<-EOF &&
+	120000 $symlink_oid	D
+	100644 $empty_oid	d/x
+	100644 $empty_oid	e/y
+	EOF
+	git rm -rf . &&
+	git update-index --index-info <objs &&
+
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 git checkout . &&
+	test_path_is_dir e &&
+	test_path_is_missing e/x
+'
+
+# The two following tests check that parallel checkout correctly reports
+# colliding entries on clone. The sequential code detects a collision by
+# calling lstat() before trying to open(O_CREAT) a file. (Note that this only
+# works for clone.) Then, to find the pair of a colliding item k, it searches
+# cache_entry[0, k-1]. This is not sufficient in parallel checkout because:
+#
+# - A colliding file may be created between the lstat() and open() calls;
+# - A colliding entry might appear in the second half of the cache_entry array.
+#
+test_expect_success CASE_INSENSITIVE_FS 'collision report on clone (w/ racy file creation)' '
+	git reset --hard basename_collision &&
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 git clone . clone-repo 2>stderr &&
+
+	grep FILE_X stderr &&
+	grep FILE_x stderr &&
+	grep file_X stderr &&
+	grep file_x stderr &&
+	grep "the following paths have collided" stderr
+'
+
+# This test ensures that the collision report code is correctly looking for
+# colliding peers in the second half of the cache_entry array. This is done by
+# defining a smudge command for the *last* array entry, which makes it
+# non-eligible for parallel-checkout. Thus, it is checked out *first*, before
+# spawning the workers.
+#
+# Note: this test doesn't work on Windows because, on this system, the
+# collision report code uses strcmp() to find the colliding pairs when
+# core.ignoreCase is false. And we need this setting for this test so that only
+# 'file_x' matches the pattern of the filter attribute. But the test works on
+# OSX, where the colliding pairs are found using inode.
+#
+test_expect_success CASE_INSENSITIVE_FS,!MINGW,!CYGWIN \
+	'collision report on clone (w/ colliding peer after the detected entry)' '
+
+	test_config_global filter.logger.smudge "\"$TEST_ROOT/logger_script\" %f" &&
+	git reset --hard basename_collision &&
+	echo "file_x filter=logger" >.gitattributes &&
+	git add .gitattributes &&
+	git commit -m "filter for file_x" &&
+
+	rm -rf clone-repo &&
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 \
+		git -c core.ignoreCase=false clone . clone-repo 2>stderr &&
+
+	grep FILE_X stderr &&
+	grep FILE_x stderr &&
+	grep file_X stderr &&
+	grep file_x stderr &&
+	grep "the following paths have collided" stderr &&
+
+	# Check that only "file_x" was filtered
+	echo file_x >expected.log &&
+	test_cmp clone-repo/filter.log expected.log
+'
+
+test_done
