diff --git a/builtin/am.c b/builtin/am.c
index 83b3d86..4f77e07 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1207,6 +1207,33 @@
 	exit(128);
 }
 
+static void am_signoff(struct strbuf *sb)
+{
+	char *cp;
+	struct strbuf mine = STRBUF_INIT;
+
+	/* Does it end with our own sign-off? */
+	strbuf_addf(&mine, "\n%s%s\n",
+		    sign_off_header,
+		    fmt_name(getenv("GIT_COMMITTER_NAME"),
+			     getenv("GIT_COMMITTER_EMAIL")));
+	if (mine.len < sb->len &&
+	    !strcmp(mine.buf, sb->buf + sb->len - mine.len))
+		goto exit; /* no need to duplicate */
+
+	/* Does it have any Signed-off-by: in the text */
+	for (cp = sb->buf;
+	     cp && *cp && (cp = strstr(cp, sign_off_header)) != NULL;
+	     cp = strchr(cp, '\n')) {
+		if (sb->buf == cp || cp[-1] == '\n')
+			break;
+	}
+
+	strbuf_addstr(sb, mine.buf + !!cp);
+exit:
+	strbuf_release(&mine);
+}
+
 /**
  * Appends signoff to the "msg" field of the am_state.
  */
@@ -1215,7 +1242,7 @@
 	struct strbuf sb = STRBUF_INIT;
 
 	strbuf_attach(&sb, state->msg, state->msg_len, state->msg_len);
-	append_signoff(&sb, 0, 0);
+	am_signoff(&sb);
 	state->msg = strbuf_detach(&sb, &state->msg_len);
 }
 
@@ -1319,7 +1346,7 @@
 	stripspace(&msg, 0);
 
 	if (state->signoff)
-		append_signoff(&msg, 0, 0);
+		am_signoff(&msg);
 
 	assert(!state->author_name);
 	state->author_name = strbuf_detach(&author_name, NULL);
diff --git a/t/t4150-am.sh b/t/t4150-am.sh
index af6053a..b41bd17 100755
--- a/t/t4150-am.sh
+++ b/t/t4150-am.sh
@@ -909,4 +909,52 @@
 	test_cmp expect file
 '
 
+test_expect_success 'am -s unexpected trailer block' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	echo signed >file &&
+	git add file &&
+	cat >msg <<-EOF &&
+	subject here
+
+	Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>
+	[jc: tweaked log message]
+	Signed-off-by: J C H <j@c.h>
+	EOF
+	git commit -F msg &&
+	git cat-file commit HEAD | sed -e '1,/^$/d' >original &&
+	git format-patch --stdout -1 >patch &&
+
+	git reset --hard HEAD^ &&
+	git am -s patch &&
+	(
+		cat original &&
+		echo "Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>"
+	) >expect &&
+	git cat-file commit HEAD | sed -e '1,/^$/d' >actual &&
+	test_cmp expect actual &&
+
+	cat >msg <<-\EOF &&
+	subject here
+
+	We make sure that there is a blank line between the log
+	message proper and Signed-off-by: line added.
+	EOF
+	git reset HEAD^ &&
+	git commit -F msg file &&
+	git cat-file commit HEAD | sed -e '1,/^$/d' >original &&
+	git format-patch --stdout -1 >patch &&
+
+	git reset --hard HEAD^ &&
+	git am -s patch &&
+
+	(
+		cat original &&
+		echo &&
+		echo "Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>"
+	) >expect &&
+	git cat-file commit HEAD | sed -e '1,/^$/d' >actual &&
+	test_cmp expect actual
+'
+
 test_done