diff --git a/usr.sbin/pw/pw.h b/usr.sbin/pw/pw.h
--- a/usr.sbin/pw/pw.h
+++ b/usr.sbin/pw/pw.h
@@ -114,3 +114,5 @@
 
 uintmax_t strtounum(const char * __restrict, uintmax_t, uintmax_t,
     const char ** __restrict);
+
+bool grp_has_member(struct group *grp, const char *name);
diff --git a/usr.sbin/pw/pw_group.c b/usr.sbin/pw/pw_group.c
--- a/usr.sbin/pw/pw_group.c
+++ b/usr.sbin/pw/pw_group.c
@@ -418,7 +418,7 @@
 	return (EXIT_SUCCESS);
 }
 
-static bool
+bool
 grp_has_member(struct group *grp, const char *name)
 {
 	int j;
diff --git a/usr.sbin/pw/pw_user.c b/usr.sbin/pw/pw_user.c
--- a/usr.sbin/pw/pw_user.c
+++ b/usr.sbin/pw/pw_user.c
@@ -378,8 +378,7 @@
 			grp = GETGRGID(gid);
 		}
 		gid = grp->gr_gid;
-	} else if ((grp = GETGRNAM(nam)) != NULL &&
-	    (grp->gr_mem == NULL || grp->gr_mem[0] == NULL)) {
+	} else if ((grp = GETGRNAM(nam)) != NULL) {
 		gid = grp->gr_gid;  /* Already created? Use it anyway... */
 	} else {
 		intmax_t		grid = -1;
@@ -1408,6 +1407,9 @@
 	if (cmdcnf->groups != NULL) {
 		for (i = 0; i < cmdcnf->groups->sl_cur; i++) {
 			grp = GETGRNAM(cmdcnf->groups->sl_str[i]);
+			/* gr_add doesn't check if new member is already in group */
+			if (grp_has_member(grp, pwd->pw_name))
+				continue;
 			grp = gr_add(grp, pwd->pw_name);
 			/*
 			 * grp can only be NULL in 2 cases:
diff --git a/usr.sbin/pw/tests/pw_useradd_test.sh b/usr.sbin/pw/tests/pw_useradd_test.sh
--- a/usr.sbin/pw/tests/pw_useradd_test.sh
+++ b/usr.sbin/pw/tests/pw_useradd_test.sh
@@ -463,6 +463,29 @@
 	    grep defaultpasswd ${HOME}/pw.conf
 }
 
+atf_test_case user_add_existing_login_group
+user_add_existing_login_group_body()
+{
+	populate_etc_skel
+
+	atf_check -s exit:0 ${PW} groupadd testuser
+	atf_check -s exit:0 ${PW} useradd user1 -G testuser
+	atf_check -s exit:0 ${PW} useradd testuser
+	atf_check -o match:"1" \
+	    sh -c "grep testuser ${HOME}/group | wc -l"
+}
+
+atf_test_case user_add_already_in_group
+user_add_already_in_group_body()
+{
+	populate_etc_skel
+
+	echo "testgroup:*:4242:testuser" >> ${HOME}/group
+	atf_check -s exit:0 ${PW} useradd testuser -G testgroup
+	atf_check -o not-match:"testuser,testuser" \
+		grep testuser ${HOME}/group
+}
+
 atf_init_test_cases() {
 	atf_add_test_case user_add
 	atf_add_test_case user_add_noupdate
@@ -503,4 +526,6 @@
 	atf_add_test_case user_add_defaultgroup
 
 	atf_add_test_case user_add_conf_defaultpasswd
+	atf_add_test_case user_add_existing_login_group
+	atf_add_test_case user_add_already_in_group
 }